branch stubs are turning black
The black coloration you see is not decay. You'll notice that the color extends below the cut, where sap dripped from the pruning cuts. The pruning cut looks like it has healed well. Sometimes harmless fungi grow in the sweet sap that oozes out to heal the cut, and this may have colored the area when it dried.
Never treat pruning cuts with 'wound dressing'. Research has shown that these products actually slow down the tree's healing process.