I believe you just have to fuse with a unit that has the same type of BB: damage, healing, support. I am not sure if the unit elemental type increases chances but it definitely doesn't have to be the same. Also, not sure if the exact nature of the BB affects it as well, such as attack all vs attack single or increase atk vs def. And as imprison said, BB level resets to 1 after evolution so don't bother raising any unit that's not already in it's final form.