Currently (Identity 1.10.1 and MC 1.16.3), there's no way for you to shoot blue wither skulls yourself, even if you are disguised as a Wither. I would like if there was a way to make it so when you are disguised as a Wither and holding a nether star, pressing the use key will shoot a blue wither skull with the ability to destroy blocks that a black wither skill can't (such as Obsidian). This may or may not consume the nether star, but it could become a viable way of mining large amounts of Obsidian or similar blocks without using a pickaxe.