The problem with that theory is that there are not really any child pornography web sites, only just a handful at any given time.
Most any that pop up are taken down within 24 hours by the FBI/CIA/Police task force.
Despite what people may think, CP doesn't get distributed over the net through websites but through usenets/darknets, file sharing and the like.
A filter is not going to be able to do anything about that.
Guaranteed that if a secret blacklist of sites is made and a filter used to block them, it will be abused by any government.