I don't think a candle can burn black I think it would just burn out. I also just wanted to say on the subject of candles what an old drama teacher said to our class once, that has always stuck with me. "Blowing out someone elses candle, does not make yours shine brighter" I think there is very much truth in this.
He was a very good teacher, one of my favourites he had lot's of passion for his work and had a very big influence on me, very well travelled. He used to quote really good proverbs all the time. Really great guy.
jonwenberg, your mostly right. What my friend says is that if you hold a burning candle in front of a cars headlights the candle will appear to burn black. I say it's just an illusion and the candle is still the same bright yellow flame.