Sun Blocks are items from The Legend of Zelda: Majora's Mask. These big, blue blocks are emblazoned with the likeness of a Sun Switch, and are placed in areas that Link has to pass through. Similar to the Sun Switches, the only way to dispose of Sun Blocks is to either shine Sunlight on them with the Mirror Shield or to fire a Light Arrow at them — both methods result in the block disintegrating, allowing Link to advance in his quest.
Interestingly, the disintegration effect is similar to those of Stones of Time from The Legend of Zelda: Ocarina of Time.