Glass candles will turn black after long periods of use because of the soot that is accumulated in the jars. Any type of candle can develop soot, although it is more common to see this happen with paraffin wax candles. You can help avoid blackened glass over a period of time by choosing bulk scented candles in jars. This will allow you to easily swap out your candles to be displayed beautifully on tables, mantels and beyond whenever black spots arise on one of them.