A black psychic is a psychic who primarily operates with 'black' or 'dark' energies. (It is not very productive nor accurate to associate 'white' with moral good and 'black' with moral evil. You can get a lot of unnecessary confusion that way.)
Black psychic energy is associated with femininity, water, night, death, peace, acceptance, nothingness. While white psychic energy can be described as offering or giving, black psychic energy is accepting or taking.
Black psychic energy is especially good for cleansing of any kind. For example, if you stumble upon a block or obstacle on either mental, emotional or spiritual levels, by using black energy it just seems to stop existing there - it was there before and now it's gone. Using white energy in the same situation, the obstacle would have to be overcome with a stronger flow which would erode the block away.
There is a kind of double standard in our modern societies: we tend to praise 'white', constructive behavior and shun 'black', destructive behavior. Of course, this doesn't work and in reality both constructivity and destructivity are out of whack. Pay attention so that you don't get caught up in such way of thinking and strive for healthy balance.
'Black' also signifies a degree of mastery. In our society there are generally more fears associated with the 'dark' as opposed to the 'light', and it can be more difficult to use and master black psychic energies in a balanced way. Open up your ego and the unknown won't be so frightening anymore.