I'm very confused by your definition of obscure. If I search for something rare and not covered very well using as set of precise search terms Altavista and company used to give me almost exclusively relevant results. Modern Google will give me windows helpdesk questions for a query about a linux driver using lots of of quoted terms, at most one of which will appear in the results. Rare results are _exactly_ the time that precise queries give higher quality hits. Substituting a different word will swamp them. Specifying sites to avoid or prefer is an extra signal to help with that