Block or Blocks in baby sign language looks like you are outlining the sides of a block. You start by taking your open hands parallel and a few inches apart. Orient your hands to outline the front & back, then the left and right side, then the top and bottom of the block. The sign for box is very similar, you just keep your hands further apart because the box is bigger.
An alternative ASL sign for 'blocks' is to form both hands into the letter 'C', then put one on top of the other, each 'C' hand taking turns until you look like you have built a tower to face height.
If your child enjoys playing with blocks, this is a great sign to teach them. Blocks is not only a motivational sign, but it can also easily be extended as your child gets more advanced. You can ask about the number of blocks, or the color of blocks. You can discuss size with big blocks and small blocks. You talk about position, with a block being on top or on the bottom.
You can use this sign with wooden blocks, alphabet blocks, or Lego blocks.