Black bolts are bolts that can only be acquired by completing a level 1 Treasure trail or by trading with another player. They may be fired by any crossbow.
Black crossbow bolts are able to harm the Tormented Wraith, when being used in conjunction with a black crossbow, though this was not always the case.
Ava's Accumulator won't pick up these bolts, however it picks up all other kind of bolts, the reason for this is unknown. It may possibly be because Black armour and weapons are not considered to be a true metal.