I also would like to request this feature.
So a black candle is formed when the stock gaps above the previous day's closing price but then closes below the opening price. So, technically, it's a red candle as the closing price is less than the opening price but what makes it special is that although the stock closed below the open but it closed above the previous day's close. What it signifies is that the traders are selling a gap up instead of bidding it up higher.
Similarily hollow candles are just the reverse of hollow. Stock gaps down but then closes above the open but the close is below the close of the previous day. Basically, traders are buying the dip. (In the pic below, the green candles i represented by hoolow candle with black outline, while the 'hollow' candle is the white candle with red outline.
Quite a few times, these are formed around turning points.