Minecraft wiki entry for Obsidian, associated text: Obsidian is, however, not immune to the wither. Although black wither skulls has merely the explosive power of a ghast fireball, blue wither skulls ignore blast resistance, which means they can destroy obsidian. The wither's body can also destroy obsidian, either through block-breaking behavior upon taking damage, or its dash attack‌[Bedrock Edition only]. The wither is the only mob in the game that can break obsidian.