@mortonbriar.
The are a lot alike. It's possible that the Smoker's Pride Black Cav. Is just BCA that didn't quite pass the visual quality inspection. The cut is fairly unpredictable as far as size goes. As for taste, it's been a long time since I bought any BCA, but to the best of my memory, they are very close.
When I first started smoking pipes, I smoked BCA (which my local B&M sold under the name Smoke Stack Black) and Smoker's Pride Black Cav. straight up by themselves all the time. I don't recall ever really noticing a difference. I just bought which ever one he had in stock.
Nowadays I mostly smoke Va's, Va/Pers, Va/Burs, and English/Balkan blends, but I always keep some of my Fat Albert and some Lane 1Q around for a good aro every now and then.
Most of the time, I'll buy a big tub of Prince Albert, and a big bag of SP Black Cav., dump them in a big bowl, cover with plastic wrap, and shake for about 3 or 4 min.
I think there's about an ounce difference in the weight of the PA tub vs. the SP Black bag, but it's pretty close to a 50/50 mix.