A blast fills an area adjacent to you that is a specified number of squares on a side.
For example, the wizard power thunderwave is a Blast 3, which means the power affects a 3-square-by-3-square area adjacent to you.
The blast must be adjacent to its origin square, which is a square in your space. The origin square is not affected by the blast. A blast affects a target only if the target is in the blast’s area and if there is line of effect from the origin square to the target.
Published in Player's Handbook, page(s) 272.