From the Super Mario Wiki
Snowman Blocks are special types of blocks that appear in Wario Land 3, Wario Land 4 and Wario Land: Shake It!. In Wario Land 3, they are purple and have a picture of a snowman imprinted on them. The dark purple Snowman Blocks can only be broken if Snowman Wario's snowball is as big as it gets. In Wario Land 4, they appear as blue blocks with a light blue border and a picture of a snowball on them. These blocks are only destroyable if Wario turns into Snowman Wario and rolls down a slope to become a giant snowball, which can then plow through the Snowman Blocks. In Wario Land: Shake It!, they are metallic in appearance and have a snowflake instead of a snowball. Snowman Blocks are usually placed to hide treasures, Coins or a Warp Pipe.