A Black Bead is one of the four colours of beads (yellow, red, white, black) that are dropped by imps. Each are required to complete the Imp Catcher quest.
After completion of the quest, the beads will continue to drop from Imps. As a result players may come to have spare beads in their banks. This gives the beads value because those who have not yet completed the quest may buy the beads in order to avoid having to kill Imps themselves. They are also collected by some players.