Black sheep of the forest?

I went for a wander around Craigmillar Castle Park this morning, before all the snow melted. This tree immediately caught my attention. It is really covered in black bark, no photo editing tricks on my part.

Any ideas on why is this one black? It looks exactly the same as other trees surrounding it, as far as I can tell.

We do have some weird trees in Craigmillar Castle Park...