In general, I don’t have big black knot problem in my yard, found one on plum in past 10 years. But I found two today on different plum trees. Out if curiosities, I cut the branch open to take a look what it did to the wood underneath.
BTW I bleached knife, cutting surface, also soaked the branch and little pieces in bleach before I toss them into garbage.
Here is what it looks like.