Is this a sugar maple?

Thank you for asking the Master Gardener Volunteers for help with your maple tree.  Large trees have a personality of their own and it is sad to see one declining.

The black bark is likely a secondary fungal infection, possibly Steganosporium ovatum.  This is an opportunistic fungus whose fruiting spores are black.  It commonly affects maples that are already in poor health for a variety of reasons--pollution, soil compaction, deicing salts, pests, girdling roots, etc.  A certified arborist (try an on-line search in your area) should be able to assess the tree and point out any issues that need to be fixed.

As far as diagnosing the fungus specifically, that would require a bark sample to be sent to a lab such as Plant Disease Clinic | (umn.edu), the University of Minnesota Extension Plant Disease Clinic.  Clear pictures, not only of the affected limb but the base of the tree, overall surroundings of the tree, and a history of the tree and its environment may all be helpful.  Unfortunately, treating the fungus is not truly feasible in many cases; supporting the tree's overall health may allow it to fight the fungus off.

 This is a link to an article on Maple Decline, a multi-factored problem that would make your tree susceptible to a fungus.  mapledecline.pdf (cornell.edu)

As far as the sap oozing down the tree when the branch was cut, that is common in maples when they are cut or injured in the spring.  Apparently there was live tissue at the cut, and the tree "weeped" sap.  This is due to an interruption in the normal vascular flow in all trees (including maples) in the spring when the trees are breaking dormancy.  This mechanism is what allows us to tap maple trees for sap to make maple syrup.

Good luck with your tree; unfortunately fungal infections usually indicate fairly serious problems.