Re: why do chemicals in a black candle clog up its wick?

Candles are made from different types of waxes. The most common wax is 
paraffin. Paraffin wax is produced from petroleum oil. Beeswax would be 
another common source for wax. Candles may contain additives which 
provide color, scent, and hardness.
Candles use a match to vaporize the wax particles which combine with 
oxygen and form a flame at the wick.
The flame provides enough heat to melt wax. As the flame burns the wax 
more liquid wax moves thru the wick by capillary action. The candle 
becomes a self-sustained process.
The candle flame has distinct regions of combustion. A blue area and a 
yellow area. The blue region defines the hydrogen combustion and the 
yellow region defines the carbon combustion. Hydrogen and carbon oxidize 
to form water and carbon dioxide.
Your question appears to be a burning problem with a black candle. 
The candle is most likely a paraffin wax colored with a black dye. The 
ability of your candle to sustain the vaporization and combustion process 
is more likely the problem rather than the color. 
***The candle wax hardness may require more heat to liquify the wax than 
the flame can provide. 
***The wick may be of poor quality, slowing the capillary action.

Some candles are meant to be burned and some are meant to be displayed.
I believe your candle would be better displayed.
Have a nice day.