You might want to post this on the citrus forum. Those folks would have the answer for you.

Sounds like Sooty Mold. This is not injurous per se to the plants, except that if it gets thick enough it can reduce the amount of light getting to the tree. It will cover both tree and fruit. It is a mold that grows on the honeydew excretions of certain insects. You can control the mold by controlling the insects. Valencia oranges usually ripen in Feb. They are one of the more cold sensitive oranges.

It does sound like Sooty mold caused by scale. Ants usually farm the scale as well. If it is in the ground I doubt it is a Velencia orange in zone 8. Are you sure it's not a Mandarin ?

I have had my Valencia Orange tree for seven years now. This year it is near 7 ft. high and has about a dozen and a half of fruit on it. Last year was it's first year to have fruit, but Ivan blew them all off. Tag says the fruit ripens in April???? But we notice they are starting to turn orange in color now. We do have it in a very protected spot. Oh, we are five miles inland from Orange Beach Alabama.