You might want to poke around this site - it will tell you more about light bulbs than you want to know. http://www.lrc.rpi.edu/programs/NLPIP/index.asp
I couldn't find anything that was specific to black light bulbs, but there is enough about T12, T10, and T8 bulbs to lead me to suggest that the only good answer will come from the manufacturer of the bulbs. It is pretty clear that there are potential changes ("performance improvements") in going from T12 to T10, and some of those clearly could affect UV output.
By the way - relatively useless factoid of the month - the T number is the diameter of the tube in eights of an inch. So a T12 is 1.5 inches, a T10 is 1.25 inches, and a T8 is one inch.