About: Black candles are used for sucking up and destroying negative energy. Often used in banishing or protection rituals.
The skull is of course a symbol of death, but in ritual may be used to represent "defying death" in some manner.
From Catherine Yronwode: Today, gamblers who practice hoodoo style candle-burning for magical purposes have been known to dress a black skull-shaped candle with Black Cat Oil, sprinkle it with magnetic sand, place a $20.00 bill beneath it as an offering, and burn it a little bit every day to increase their chances of bringing in the winnings on long-shot games like the lottery. The idea behind this ritual is to openly acknowledge and then to refute repressive Christian prohibitions against gambling, which warn believers that playing games of chance will lead one to an early death and thence to Hell. By calling on the black skull for gambling luck, the player acknowledges "sin" as a way of life and laughs in the face of religious condemnation.
*Disclaimer Metaphysical claims are based on lore, history, and personal experience and should not take the place of professional medical care.