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 4, they appear as blue blocks with a light blue border and a picture of a snowball imprinted on them. These blocks are impenetrable; not even a bomb can break it. The only way to destroy a Snowman Block is by having Wario turn into Snowman Wario and roll down a slope to become a giant, uncontrollable snowball, which can then plow through the Snowman Blocks.
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: Shake It!, they are metallic in appearance and have a snowflake instead of a snowball. Snowman Blocks are usually placed in strategic places and often hide treasures, Coins or a Warp Pipe.