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

C < O

A Short Black Candle is also a Short Candle. The following formula checks for the body to be more than 50% shorter 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 < AVG(ABS(O - C), 15) / 2

Combining these formulas results in the following.

C < O AND O - C < AVG(ABS(O - C), 15) / 2