This would work if you had a blacklist of domains you didn't want to see. But the idea in the post is closer to a whitelist: the highest priority sites (tier 1)

This would work if you had a blacklist of domains you didn't want to see. But the idea in the post is closer to a whitelist: the highest priority sites (tier 1) should be set manually, and anything after that should be weighted by how often it's referenced by the tier 1 sites. For a lot of searches you're going to have to pull many results to fill a page with stuff from a small handful of domains, and in fact you might not be able to get them at all. And that's before you start dealing with the weighting issue, which would require quite a bit of metadata to be sent with each request.