You know on any given day that I spend using blox (or nep for that matter) I run into at lease one instance where my life would be easier if I could specify "Not" for a search option.
For example show me everything that's not wire... show me everything that's not sports... show me everything except breaking or developing.
I understand your UI design goal is to make it so easy a trained monkey can do it but how hard would it be to modify check boxes...
Empty (not selected)
Click - Checked (selected)
Click - Red X (search excludes this item)
Customer support service by UserEcho