Are there special rules in SCRABBLE for playing the blank tiles?
The official rules of SCRABBLE do not allow a player, later in the game, to change the letter that the blank represents. Likewise, a player may not replace the blank with the letter it represents and use the blank in another word. At the National Scrabble Association’s clubs and tournaments, this rule is strictly enforced. The National Scrabble Association also advises that to avoid later confusion, both players record on his or her score sheet what letter the blank represents at the time it’s played. Those of you who’d like a game with more scoring chances may play the variation that allows players to replace a blank on the board with the letter (from your rack) it represents. The drawback (or advantage) to this version of SCRABBLE is the increased luck factor.
