The Black Hole special sucks. Literally. When activated, the innermost ring of letters surrounding the special will get sucked into the special and scored. All other letters on the board get sucked in by one row, and spun around the board 90 degrees. Any that end up off the board or find themselves on a cell already populated get unceremoniously dumped with no points awarded.
Hint: a Black Hole on or near the edge of the board will result in a cleared board. You’ll only get points for those letters in the innermost ring, but this will free up space on what might have been an otherwise crowded board. This is especially useful when you get up in levels and are required to spell 4 letter or greater words on a very small board.