Yeah, 12/8 works for slow blues. It has a sort of triplet effect as in, it's counted (usually) in a 3 3 3 3 form. Although some composers for band and orchestra pieces decide to be complicated and make it something like 2 2 2 3 3 or something dumb like that. A song, Vesuvius, which my band played, uses 9/8 a bit, and is in a 2 3 2 2 form. And yes, 12/8 doesn't distinguish speed. Bpm's do. So yes, simple answer.