Shameless plug, but I've been working on a project [0] that does exactly this. Currently it just has a few filters I've created for myself and only supports web search (and a few !bang like re-directs), but I'm working on implementing user accounts that will be able to create their own filters.
Yup, that's exactly it. I've found that there are certain websites that I have basically no interest in seeing. Eventually I might incorporate more granularity to get sub-domains or something like that, but for now it's just whole websites based on URL.
I was pretty sure that it could be done in Google with operators in the search box (going back a few years), but I don't use Google any more and one reason I stopped was that it kept incrementally degrading the ability to refine individual searches manually. Anyhow, I just did a DDG search and came across this [1], which looks interesting for your use case (although that Pinterest is mentioned is a coincidence). I've not tried it out, so I can't recommend, comment or anything.
I'm well aware of the various search flags. I can also think of at least 10 domains I'd like to permanently obviate from every search. Adding flags for all of these every time is unwieldy. I have toyed with browser extensions to achieve this, but I quickly learned that using many of these flags will compromise search results. A good solution will require a search engine that anticipates this use case.
I'd actually pay nominal amounts of money for a search service that had my interests in mind; as opposed to advertisers and thought police.