I like the idea, but does it come with a default list? I'd be interested in something like this but that includes most known blogspam sites, with a global list like say pihole provides.
High-ranking search results don't come easily, and there will be a relatively small set of domains occupying any such slots. Even with turnover, that's going to be viable. Remember, the competition here isn't to flood your inbox from any random domain on the Net, but to secure one of the first 10 SERP results. Odds are good that blocking 10--20 domains will be more than enough.
Your list will also likely vary from other people's.
I wish there was a predetermined list with things that are known to be poor results, like pinterest on google image search.
In practice it should be a winning battle, since it takes time and money to spam site up in ranking and low effort to blacklist it.
I will try this out.