valencia orange trees
i have 2 Valencia orange trees. Both of them have very dark fruit. Both trees have fruit that starts green, seems to turn almost black and doesn't seem to ripen. Blossoms appeared about Feb. and fruit is still on the trees.
Valencia fruit is generally ripe and harvested between Mar thru Oct.
The black that you describe sounds very much like Sooty Mold.
I have listed links for you.