The black powder on top are the spores which the mushrooms release after having matured. This is very normal.   It is best to harvest them before they start releasing the spores, because the spores can cover the grow kit so much that new pinheads cannot grow from it anymore. If there is a thick layer, then it is best to soak the cake completely underwater and to very gently and carefully rub the spores off with your fingers. Careful because it is very delicate.