A Long Black Candle is first off a Black Candle which closes below the open.

C < O

A Long Black Candle is also a Long Candle. The following formula checks for the body to be more than 50% longer than the mean body size over the most recent 15 candles. It can use O - C instead of ABS(O - C) at the beginning of the formula because we know C < O.

O - C > 3 * AVG(ABS(O - C), 15) / 2

That is actually the final formula because if it is true, then C < O also has to be true.