Read it in Steve Nison book of candlestick patterns which is supposedly one of the best
Also this is mentioned in http://www.investopedia.com/terms/d/darkcloud.asp so yea you need 3 candles minimum. 2 is no guarantee it is a successful pattern which is why I mentioned we need 3, like Nison does too
ps: You are right that they are strong reversal signals though, if they are confirmed.