Minecraft wiki entry for Wither, associated text: Black wither skulls explode with a blast power of 1, the same as a ghast's fireball, and cannot break blocks with a blast resistance above 4.
Blue wither skulls have the same explosion strength, but move slower and are more destructive to terrain. Blue wither skulls break all breakable blocks including obsidian, ancient debris, and blocks of netherite), but cannot break unbreakable blocks like bedrock and end portal frames. In Education Edition, blue wither skulls deal increased damage, but with the same explosion radius.