Congratulations on having a safe encounter with the Blacktail. I’ve found them to be quite common in the Davis Mountains. Fortunately, they are generally much more “laid back” than the Western Diamondbacks and Mojaves that one encounters at lower elevations. Keep an eye out also for the much smaller Rock Rattlers; they blend in with the substrate very well and are even more abundant than Blacktails. Interestingly, the Rock Rattlers from the Davis Mtns are highly variable in coloration and pattern; they seem to look different in every canyon.

As for your question about the Whiptail lizards of that area, I must admit that is one group that I have yet to get a handle on. There are eleven species in Texas (mostly in the Trans-Pecos) and more than half of them are unisexual (parthenogenic), most having arisen via hybridization between formerly allopatric sexual species. They are basically the Empidonax flycatchers of the herp world; I’ve seen even lizard specialists reach for the keys when trying to I.D. them. They are also a favorite among herp taxonomists and consequently the group is in a state of flux. In fact, the genus Cnemidophorus has been recently split, the US species now enjoy the generic moniker of Aspidoscelis (the current round of field guides don’t include this yet). I also have seen some really large ones in the Trans-Pecos, most likely are the Tiger Whiptail (A. tigris) and/or the Checkered Whiptail (A. tesselatus). As you no doubt know, they are very difficult to catch or photograph.

Nice photos of the Blacktail.