RuneScape Classic Wiki
RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Black Candle
The Old School RuneScape Wiki also has an article on Black Candle.

A Black Candle is used during the quest Merlin's Crystal. You acquire it by giving Wax Bucket to Candlemaker in Catherby. It can be lit with a tinderbox, like a regular candle.

It can be kept by the player after completing the ritual in Merlin's Crystal. It may not otherwise be obtained after the quest has been completed.