It was a tool to remove a site from your own results. If you searched and saw a site you didn't like, you could click to no longer get the results from that site. It wouldn't affect what anyone else saw anywhere.
That's a good point. I guess bad actors (exactly the same ones that publish those shit-sites) can abuse it to block the actual sites.
Not a problem as long as it only affects the user themself?
My guess is some ux head exploded when they realized someone might blacklist a site they wanted. Or maybe the one who made it got promoted and no one stepped up to maintain it?