A black candle is a Firemaking light source used in the Merlin's Crystal quest.
During Merlin's Crystal, in exchange for sparing Sir Mordred's life, Morgan Le Faye details how to free Merlin, requiring a black candle among other things. The Candle maker in Catherby offers to make one if the player brings him a bucket of wax. Once all the other items are obtained, it is lit with a tinderbox when inside the magical symbol, inside the fence on the north-east side of Camelot Castle. After the remaining items are used, and with the correct incantation, Thrantax the Mighty is summoned for the player to command to free Merlin.
Black candles can not be purchased from the Candle maker after Merlin's Crystal, but multiple can be obtained during the quest. Although they aren't tradeable, they can be put in empty candle lanterns to make candle lanterns, which are tradeable.