Looks painful. That large black area may be a drainage site or point of injury site. It may not be bumblefoot, it could be an infection from a splinter, snake or spider bite, or other injury. I would remove the scab and see what's under it before I go cutting into the sole of the foot.
If it all looked like muscle, then there are some possibilities there. The apparent muscle may be fibrous material or it may indeed be muscle, but it's pushed outwards due to something behind it. So, in this case it could be bumblefoot inside, or it could be an encapsulation of an infection or foreign object, cyst, etc. Personally I would bet you are looking at a lot of fibrous material with muscle somewhere in the mix there.
You'd need to cut deeper, it looks like. You could just try the direct bumblefoot approach, cutting into the sole of the foot, it may yield results. Very odd that there is such a large scab with nothing behind it, maybe it was a drainage site where fluids escaped a while back.