This appears to be black sooty mold.  It is caused by a fungus and is not harmful unless it is so extensive it causes reduced photosynthesis. It usually is accompanied by honeydew production from insects.  It can be an indicator there is a insect problem. If it is aphids or other insects they can usually be washed off the trees with a hard spray of water.  ( Aphids can’t fly or climb back on the tree.) Since your apples are doing well it is probably not necessary to treat it.  Read more about it at the following websites.
1. https://extension.umn.edu/trees-and-shrubs/non-harmful-tree-conditions
2. This reference gives a good overview of how it might be a problem depending on the insects involved.
https://www.arborvisioninc.com/tree-treatment/black-sooty-mold
3. https://extension.umn.edu/yard-and-garden-insects/scale-insects