Small Shine Block
From the Super Mario Wiki
Small Shine Blocks, are objects found in Mario & Luigi: Partners in Time. They are semi-rare blocks with a Shine Sprite symbol. These can only be hit by Baby Mario or Baby Luigi, as they are the only ones that can enter dark rooms. It is impossible to access these blocks in the room unless an Adult Shine Block is hit first. The blocks release a Shine Sprite that illuminates the dark room. After struck, the block disappears.