The acid in the wood does that just like wet Oak,Walnut ect any wood with tanic acid. Use Real Lemon Juice to take the black off your hands and tools. I keep one of those little plastic lemons on the shelf behind my lathe cause I turn allot of green wet wood. I believe it was the late Doug Barnes that told us to use it at a class at John C.Campbell Folk Art School. Doug was an outstanding turner,instructor and friend.
It's like magic how easy that black comes off with the juice.