Same as Washington, Oregon and California
Mulies have big white bums with a bit of black on the end of the tail, sometimes a skinny stripe going up the tail and the tail is kind of "ropey". They are larger than blacktail on average. Antlers are more open. They prefer more open high country further inland, most often migratory, when startled they bounce like some exaggerated springboard.
Blacktails have a more stubby tail, black or gray, cagey antlers often pointing more inward, no big white rump. They like thick, steep coastal rain forests (think mossy terrain with ferns all over) and unless migrating out of necessity or cruising for babes on average they stick to a fairly small home range. They don't bounce like muleys when spooked, but can to a degree, or a quick trot, or just run like a race horse... often they won't flee at all. They are masters of hide and seek game in the best terrain to play it. They can almost seemingly disappear before your own eyes in the timber. Like, what? He just walked behind that one tree and that he was gone? Like there was a magic portal behind the tree? What the heck?!
BT's for the most part are found within 200km inland in the west coast of North America as well as the coastal islands. To confuse the matter there are Sitka Blacktail and Columbian Blacktail. And where Mule Deer and Blacktail range overlap, any ratio of hybridization is possible. I believe this may also be happening in places blacktail and whitetail increasingly overlap, but that is a whole other thing. The of prevalence of hybridization between blacktail and mule deer is the reason they both fall under the same tag as running a DNA test before the shot isn't too feasible. In their pure forms, blacktail and mule and two very different creatures with much different behaviour and physical appearance.
I hope I got that all right or mostly right. All I know is that I think blacktails are the coolest critters on the planet. I would guess both these bucks below have some level of hybrid in them, but by appearance one is clearly blacktail dominant while the other is mule dominant. Worth noting they were both taken within about 50 yards of each other.