What are the rules for playing a blank tile?

Although some players use house rules to govern the blank tiles, the official rules of Scrabble do not allow you (1) to replace a blank tile with the letter it represents so you can use the blank in another word, or (2) to change the letter a blank tile represents.

At official tournaments, this is strictly enforced. In order to avoid any confusion, the National Scrabble Association recommends that both players make a note of what letter a blank tile represents when it is played.