Why is the gravel underneath my birch tree turning grey/black?

Sounds a bit odd - have you examined it closely, the gravel I mean? It could be some kind of fungal thing building up in the gravel, or perhaps the tree has been dripping "sticky" (aphid infestation?) onto the gravel beneath and the dirt in the air has stuck to it.

Might be best to use bark chips underneath the tree - but make sure there's not a more serious problem going on first

I have an area of gravel next to an old tree trunk I couldn't get out in our conservatory. Every so often, after a good watering, it turns jet black and lots of little fungi come up. I guess there's something under you gravel that's feeding your fungus, as I guess it's similar. Could be the leaves from the birch decomposing with fungi. (All part of Nature's way, and nothing to worry about, I'd say)