From your original comment: > Third, some people will probably block domains they shouldn't have blocked, and then have a bad user experience in future searche

From your original comment: > Third, some people will probably block domains they shouldn't have blocked, and then have a bad user experience in future searches as the sites with genuinely best results is blocked. And then you're only left with only bad options: ignoring the users' stated preferences which they'll hate, or serving bad results that they'll also hate. That's a real possibility, but it can be addressed by presenting a block that would say, for example "results from blocked domains" or some such.