BL isn't based on usage; it's Pokémon who dropped from OU to UU and then were determined to be too powerful for UU (most recent examples: Cresselia and Porygon-Z).It could go into BL but the problem is that BL is so small that it's hardly a tier and everything in BL is pretty much stuck to being used in OU. If 35+ Pokemon are in BL, I guess it could be a full-fledged tier. Right now we have 9. That's nowhere close to running on its own. OU has 49 and UU has 54.