While the black cat image has long been considered a bad luck sign, the burning of a black cat candle is intended to reverse bad luck for good. 

Write the desire for good luck wished for on a piece of parchment paper and place beneath the candle.  Light the candle, burn for 7 minutes the first night, 11 minutes the second night, then 7 again the third night, 11 on the fourth, etc.  Alternate each evening the burning for 7 or 11 minutes until enough good luck  has been received.