I don't think BKB was used because of the special, which is Devil (not very good compared to Demon)...
But yeah Blood Tornado outclassed BKB for that reason (It is Demon and very strong), plus, Bloody Art is very easy to get compared to MKB.
BKB was the strongest Double Saber on PSO, until Inferno Girasole was released. Girasoles are also much easier to get with good %s than MKB.
Anyways, all of those weapons are pretty much useless because Psycho Raven/Demon Mechguns are so much much better, easier and safer to use (since they are ranged)... For someone to care using them, they would need the double of ATP they currently have.