OK, just to clarify---
Blacktails are SOMETIMES considered a subspecies of Mule Deer, (not the other way around), but some studies and research do not believe that to be the case and rank them completely seperately. They are typically smaller than either Mulies or White tails, but their antlers fork like Mulies and they run with the same kind of gait, which is a pronounced "bounce" with all four feet leaving the ground. They are typically broken into 2 subspecies of Blacktails, Columbian and Sitka.
Mulies and Blacktails ARE more closely related to each other than to white tailed deer. Mulies are certainly more widespread than black tails.
Wikipedia has some good info, as do several other sources, but you have to tease it out!