We already referenced this problem in 1997 in a paper:

If a huge number of computers or individual pages is blocked, neither the individual operator of a computer nor the individual user will know whether there is a technical defect that can be eliminated or whether some content has been blocked.

http://kris.koehntopp.de/artikel/blocking/index.en.html

The 451 does not really fix this problem but for a protocol and application specific implementation.

Funny thing is: government paid us to write this paper but hasn't understood it even 15 years later.