Here’s my ted talk on how black dragons evolved to fill their niche as primarily cave dwelling creatures :’)
They tend to be longer in the body and not quite as bulky as they’re portrayed to be in game. They’re nothing like as long as the noodly Cloud Serpents of Pandaria, rather it’s more like a ferret which is absolutely what I’m basing this on lmfao. Black dragons are essentially just big scaly ferrets in terms of their build, though not quite that long. In line with this they have much, much shorter legs than other dragons comparative to their body length, and walk with an arched back when above ground. When in caves/tight spaces they walk with their backs straight, dropping their height and allowing them to fit into tighter spaces.
Their shorter legs allow them to retain full degrees of mobility when moving in tighter places, meaning they’re essentially as fast bellow the ground as they would be above. All of this makes them bendy as fuck though, with whelps especially just sort of acting like slinkies you can bend in lots of weird ways without hurting them. Adults too can bend themselves into weird as hell position that shouldn’t be considered possible for something their size, and to a dragon from another flight certainly wouldn’t be as a rule.
Their bodies are much, much hotter than other dragons to the point their scales canonly retain heat long after the dragon itself is dead/the scale is removed. They’re comfortable wallowing in magma, indeed their preference tends to be to partially submerge themselves in it when lounging around. Even in mortal forms they run much warmer than normal.
They’re very cold averse though being that they tend to gravitate more to volcanoes etc for dens, and as such do generally avoid colder climates where they possibly can. Their eggs require higher temperatures to incubate as a result.
Generally black dragons aren’t as big as dragons from other flights. Neltharion was an exception, and ultimately Wrathion would also be an exception as he’s going to be a fairly hefty boy.