Of the four species I guided for in my lifetime black bears are the most difficult to track for a variety of reasons (and to keep a long explanation short I need not get into those details).
Although not absolutely essential for tracking, blood sure made recovering them easier and quicker. For that reason I would always recommend my hunters use three blades for black bears.
I don't like talking numbers, but for the sake of my opinion meaning something, and to explain as to how I reached that conclusion, let's just say that very few people have handled the quantity of black bears that I have.
I like two blades for moose, but I killed several with three blades. Whether hunters used two or three blades for caribou or deer, it didn't matter to me, as long as they were truly sharp.