Blue Shake King Block

From the Super Mario Wiki

The Blue Shake King Block is a type of block that appears in the game Wario Land: Shake It!. At first these blocks will be transparent with the Shake King's face located in the middle and blue dashed lines bordering the block. However, once Wario frees a Merfle from its cage, all the Blue Shake King Blocks will become solid, turning into Red Shake King Blocks. Additionally, the Red Shake King Blocks will turn transparent during that incident, becoming Blue Shake King Blocks.

Blue Shake King Blocks tend to be placed in areas that Wario can't re-enter, which means that he'll have to find an alternate path to get back to the level entrance.