From the Super Mario Wiki
Uncertain Blocks are a special type of block exclusive to the game Wario Land: Shake It! Uncertain Blocks appear in two different forms: the first being solid and colored and the other being a border of a block. The only way to change Uncertain Blocks is by hitting a Certainty Switch of the corresponding color. There are three colors: red, blue, and yellow. These blocks can either impede or help Wario, depending on the situation. They can be used as platforms when solid.