**This isn't an intentionally blocked name by Mojang, it was due to a bug/overlook.**
I'm sure you've heard about the Minecraft infinite alt accounts exploit, but if you didn't there's a link at the bottom explaining how it worked. Eventually it got patched though, and all the accounts got deleted. However when trying to change your name to one of the deleted accounts, for example the name "SpergOut", it still claims that a profile already exists with that name.

https://www.youtube.com/watch?v=_ChOe_cXcoQ