I hope that works! It should do something. Keep us posted! Someone else may just come along with an idea or insight too.
By the way, welcome to BYC!! :frow
Bumblefoot almost always has a black scab and whilst it's hard to see on a black foot, I don't see any, yet, what else could it be? Maybe it's an early case of it.
Does it feel hot to you and is it hard or soft? If it's warm/hot, and whether hard or soft, there's probably some infection in...