It's very common. Assuming the trees are otherwise healthy, mature Sugar Maples frequently develop this black color. It is cause by a mold that does not harm to the trees.

Arktrees

need some pix.. check out the link

could be maturity as ark seems to suggest

or could be honeydew from aphids..

ken

Here is a link that might be useful: link

IMO, the black bark gives a fantastic contrast to the fall color display WRT sugar maple.

"could be maturity as ark seems to suggest"

I think Ark was suggesting the same thing as you are, Ken. The black sooty mold grows on the honeydew (which is not, itself, black). I personally don't care for the look, but TEHO (since we are using abbrs).

Yeap, Brandon/Ken. That's what I was saying. I just did not want to go into detail unless the OP wanted it.

Arktrees

Another common possibility is sapsucker damage resulting in sap leaking down the trunk and a black fungus growing on the sap. If you see rows of holes in the trunk above the blackened area, that is probably what's going on. This is usually not a big problem for the health of the tree unless the bird damage is extensive.