|In Basic Edition, blink dogs were said to resemble dingos|
As always, let's begin by seeing what the primary source material has to say about the creatures, using an admittedly incomplete sampling of various editions:
First EditionBlink dogs are said to be yellowish-brown in colour, and to stand about 3 feet (90 cm) tall at the shoulder, which would make the average blink dog the size of a particularly large wolf. The picture shows a heavily built, short-limbed, animal, with a relatively short, wide snout, reminiscent of many domestic breeds (and, to be fair, some extinct species) rather than any real-world wild animal. The tail has a bush only at the tip, rather than along the length as is typical for most wild canids; it may be intended to resemble that of an African wild dog (Lycaon pictus), which would match the original description given way back in the pre-"First" edition supplement Greyhawk. Other than this, blink dogs are harder to injure than regular wolves or domestic dogs (possibly due to nimbleness and skill, rather than a thicker hide or greater mass), but have a bite that's broadly similar to that of a wolf. In addition to their obvious signature ability and human-level intelligence - about which more below - there is also some limited information on their pack size and composition.
Third EditionThe fur is still said to be yellowish-brown in this edition, although at least one of the illustrations shows it be a relatively dark shade of brown, perhaps suggesting some intra-specific variation. The ears are tufted, like those of a lynx, and the animal also has a short beard and rather odd projecting eyebrows, but this time the tail is bushy like those of real wild canids. They are stated to be omnivorous. Compared with wolves, they both have a thicker hide and a greater natural agility, although, surprisingly given their heavier build, they lack a wolf's physical strength and hardiness. For no obvious reason at all, they are also said to be able to "see" in the total absence of light.
Fifth EditionThe illustration now shows an animal with the shape of a great dane, and with elongated ears that either project upwards naturally or are floppy and being flung upwards by its motion. Its fur is extremely short, if it even has any at all, and the colouration is shown as a dark yellowish-brown. It still has the beard, plus some strange projecting spurs on the ankles and wrists. As in First Edition, the tail has a tuft only at the tip. While it is still more agile than a regular wolf, it now has similar strength and endurance, and actually has a thinner hide (possibly due to the thinner/absent fur). We are, as is standard for this edition, no longer given any information on the size or composition of their packs, and there is also very little information on their habits or biology.
From this, we can say that blink dogs are probably the size of large wolves, but more closely resemble domestic dogs, although exactly what kind of dog is less apparent. Like wolves, they can run fast, and seem to be pursuit predators. This also fits with their stated preferred habitat of open terrain, such as temperate grasslands. In those editions that list such things, they have a highly sensitive sense of smell, and acute hearing - which would explain the large ears. It's possible that they also have good night vision, suggesting a nocturnal lifestyle, but this is not universally apparent.
All in all, there's little to suggest that they aren't, biologically and anatomically, very similar to real-world canids, with a stronger resemblance to the wolf-like kind (wolves, coyotes, jackals, dholes, etc.) over the fox-like, where there's a difference. When it comes to their habits, however, the main difference is likely to be that all editions agree that blink dogs have human-level intelligence.
Obviously, blink dogs do not have hands, so whatever culture they have must be a purely oral one. In most editions, they are said to have a language consisting of "barks, yaps, whines, and growls". Fifth edition adds that they can usually understand the Sylvan language of fey beings, although the structures of their mouth and larynx would prevent them from speaking it, or any other humanoid tongue. Given their intelligence, their native language can presumably convey just as much information as a human one would, although its vocabulary would focus on their own interests. For instance, they would likely have many words for scents or smells that wouldn't translate into Common, and lack words that refer to specific types of artefact such as, say, distinguishing different kinds of weapon.
Pack size varies slightly between editions, but around ten adults seems typical. This is much larger than a regular wolf pack in the real world, implying a high degree of sociality. Where a real wolf pack normally has a single mated pair and some near-adult children, plus pups, the large packs of blink dogs suggest that two or three mated pairs per pack is the norm, rather than an exceptional circumstance. Blink dogs are stated to be Lawful, so there is presumably a clear hierarchy within the pack, which is probably on the basis of experience and age. This would mean that there would be few challenges for dominance, with younger blink dogs having reverence for their elders even if they no longer remain in peak physical condition. Maintaining oral traditions down the generations is likely a key element of this.
In addition to the adults, though, First Edition also states that a pack typically includes around seven pups. If we take this figure literally, it implies a fairly high infant mortality, although, since its hard to see what sort of medical skill a dog might have (beyond "eat this, it'll help"), it's perhaps not unreasonable. It also supports the idea of there being multiple mated pairs in the pack, unless litters are larger than they are among wolves; I'd suggest that it's more likely they're smaller, say, around three or four each. Because of the greater brain development, pregnancy may last longer, too; three, perhaps four, months seems reasonable.
To maintain genetic diversity (assuming that's necessary in a fantasy world), adults must move between packs when they are old enough to mate, but their highly social nature may mean that "lone wolves" are uncommon at best. Instead, packs must meet up from time to time to share stories and initiate romances between unrelated individuals. Although they surely have preferred home ranges, likely of over a 100 square miles per pack, their generally good natures would also imply that they aren't highly territorial, and that interacting peacefully with one another is the norm. They're probably monogamous, and to a greater extent than wolves are.
Given their native habitat and shape, blink dogs almost certainly have a similar diet to wolves, with the addition of berries, shoots, and tubers to reflect their stated omnivory and ability to apply their intelligence to identifying new food sources.
All of which brings us, at long last, to the reason that they're called "blink dogs". This is due to their ability to teleport, vanishing from one spot and instantly re-appearing in another. The exact details, however, vary between editions. Originally, the ability is only vaguely defined, and appears to be mostly defensive, being used as a means to avoid being struck. In First Edition, it is used tactically, with the blink dog frequently re-appearing behind its target to attack it from that angle. In Third Edition, we're told that the range of the ability is over 700 feet, and that it can be done repeatedly, while in Fifth, the range is only 40 feet, and it can take a (usually short) time to recharge between uses. Third Edition also adds the additional power that blink dogs can rapidly flicker between physical reality and an invisible/incorporeal state, so that some attacks randomly pass through them without harm.
Obviously, the exact details of how they could to this are irrelevant: it's magic. The way it is used also seems reasonable, since blink dogs are intelligent enough to be able to employ quite sophisticated pack tactics to distract and surround opponents. In a natural setting, this would be used to capture prey, perhaps making it easier to separate an individual deer or the like from its herd - simply teleport into the middle of the group, lunge at an individual to make it dart away, then vanish before its neighbours can retaliate.
The version as described in First, Second, and Third editions is also a handy means of escape, and allows blink dogs to traverse great distances very rapidly. This is a further reason why they aren't likely to be territorial, and it's possible that they might have feeding sites, or the like, a long distance from dens where they rear their pups or other spots of safety. In this case, even a hundred square miles for their home range may not be enough - or, indeed, be largely meaningless.
As an aside, this use of blinking for travel doesn't work in Fifth Edition, because the range is short enough, and the recharge time long enough, that they could more quickly traverse the same distance by running. Having said which, if it's less exhausting than physical movement, it could be used in extremis to migrate to an entirely new locale.
Finally, it's usually said that blink dogs are particular enemies of displacer beasts. It's not really obvious why this should be. They have different habitat preferences, so they won't compete much for resources. Displacer beasts are individually more dangerous than blink dogs, live in groups that aren't that much smaller, and have defensive abilities that should be fully effective against them, so it isn't an easy fight. It seems to me, therefore, that this is an enmity that's specific to a particular game world, in which the creatures have some shared origin that provides a specific reason for their mutual hostility.
[Photo by Sam Fraser-Smith.]