Comment Re:It would be an error code (Score 3, Informative) 255
Just to clarify, if a web site is being blocked, then that web site can not send an error page to the client making the request.
The error would come from whichever device is blocking the web site, and it would prevent forwarding of any data packets to the blocked site. The blocked site can't return an error page because it has no way of knowing someone trying to access it was blocked. Whatever device is doing the blocking is the one that can send an error code, if at all.
Returning an html error page would be entirely optional, and I seriously doubt whomever is doing the blocking would give a rat's ass about a fancy custom error page. If they did, it might make for a nice amplifier in a DDoS attack.