I have had a few people ask why their candles turn black when doing magick. There are a number of reasons why. It could be because you have forces that are interfering and don’t want you to move forward, someone sending negative energy towards you or you just need a cleansing of yourself as well as the space you live in.
I would suggest that before you do any magick, take the time to cleanse and protect yourself and your space. Clear your candles of all energies before you charge them, hold your candle with your power hand (the one you use them most), then charge it with your intent. Sit the candle down and PROTECT IT. You can protect it by putting salt around the base of the candle also light a white candle to protect the magick, gemstones and whatever your intuition wants you to add. From my own intuition I burn white sage in front of the candles.