My flame turned black. Never had a flame turn black before and read …
“ If a flame burns black, be warned. A black candle is indicative of negative energy or spirits. These flames hold darker meanings of mystery and peril. A dark presence or misfortune may be near.” from How to Interpret Candle Flame Meanings: 12+ Spiritual Signs.
I agree with @tracyS here - check the mundane stuff first! A lot of things can interfere with candles, and candles are notoriously fickle anyway. The slightest difference in candles while they’re being made can make them act differently when they’re burned.
I would say if it happens more than three times with different candles then it’s probably a sign of something. If it’s just a one-off thing then it’s probably a fluke!