ISP blocking access in this way acts as a proxy, and proxies are servers themselves or act on behalf of the servers so they use 5xx codes for their errors. Now using code in 4xx range for censorship actually shows that your ISP haven't done this unwillingly only because they were forced to do so. They tell you that YOU are in error by even sending this request, they say it's illegal for you to request this page. Is that right? Tomorrow you'll be forbidden to even think about it.
You see, choice of code in 4xx or 5xx range in case of censorship is an ideological issue, not a technical one. I'd use 5xx, because it's a proxy's problem that it cannot fulfil the request even though it was deliberately set up just to refuse it. I don't like 451 only because it lies in 4xx range and Tim Bray's I-D should be revised unless IETF advocates censorship itself!