Blackened color on apple tree pruning wound

I reduced height of my apple tree and noticed there are blackened spots on 2 year old pruning. What kind of disease is this?

Black rot?

It appears to be only going after the wood and fungus always goes after the wood of pruning wounds, no matter the color. I only worry about fungus that is able to attack living tissue. Trees wall off attacks on dead wood and healthy trees don’t have much problems surviving even very large pruning wounds. Callous was rapidly forming to close those wounds without any interference from that fungus, at least in the first photo. Maybe you left a stub so the tree couldn’t protect the wood on the second photo. That’s why its a good idea to cut back to a vigorous shoot and not leave any stubs. Stubs tend to rot.

I believe only a plant pathologist could accurately identify that particular fungus and with a sample and a microscope. I believe many different species have a black color.