For many years the Blacktail Deer has been considered a subspecies of the Mule deer, however recent DNA testing has proven this not to be the case. In Valerius Geist's book Mule Deer Country he explains that by testing the mitochondrial DNA (the mothers DNA ) of the three species (blacktail, whitetail and mule deer), researchers have now determined that it was the mating of Whitetail does and Blacktail buck's that gave rise to the Mule deer and not the opposite as was once suspected.
It is now believed that millions of years ago the Whitetail deer expanded its range down the east coast of the United States, across Mexico, and then back up the West coast, where it eventually evolved into the Blacktail Deer. This may help to explain the strong resemblance in appearance and psychological characteristics between the two. Thousands of years later as the recently evolved Blacktail's range spread eastward and the Whitehall's range again expanded westward, the two deer again met. At this point the Blacktail bucks, displaced the Whitetail bucks, and bred the Whitetail does. Researches now believe that it is this hybridization that produced what is now know as the Muledeer.
I think I found some info on the DFG site a few years ago too. I had a place I horn hunted east of Roseville. That was always a question I had too. I have some monster sheds I've found and never took the time to score them because of this. Hope you find the info your looking for.
Blacktail bucks displaced Whitetail Bucks? I guess thats just a hard theory to grasp, considering the problems Mule deer bucks are having being out breed by whitetail bucks, not disagreeing with what you said or anything just seems odd to me.