Oh- i wouldn't expect retractive search functions to be any more perfectly accurate than any OTHER search function ever is, because of poor or false tagging, but never the less, it'd save a load of time and trouble to be able to search for something that is, for example, a word with multiple meanings. You would be able to search for Blade, NOT sword, or promo, NOT demo, or to remove most mesh results, since most of those are tagged as Mesh now. Much as I'd love to see this feature, the other - the ability to remove particular shops or merchants' results from a search would be much more valuable. I love to browse lots of results, but nothing is more of a turn off than nine pages of identical t-shirts in 400 colors, and likewise, it's fruistrating when you've already gone through a particular merchant's whole collection and you know they don't carry what you need, to not be able to simply remove that shop from your search.
you also say "Given your example, you go to Apparel>Women>Footwear>Boots, then search Black within those results. In essence you have what you need already..." Not so. I would be able to do that if I WANT black boots- but not if I DON'T want black boots. If what I'm looking for is any color that ISN'T black. Also, you mis-tagged the header of the post- my comment/suggestion isn't about irrelevant results or tags. it's about IMPROVING the relevance of a search.