Edit: To save anyone the trouble: As of late 2017 this is simply not possible. That's been confirmed to me by Trello support themselves.
Edit for clarity: I want to "hide", or "remove" specific cards using a filter. Currently you can only "show" cards. At the moment, each thing you select in a filter hides everything else and shows that thing. Select blue: you see only blue cards. Select also red: you see both blue and red cards. But I want to be able to say "Show blue but hide anything labelled red".
I have a Trello board with cards which can be labelled "red" or "blue".
I want to be able to filter so that I can show cards which do not have the "red" label.
So I'd see:
Blue labelled cards. Cards without a label.
Red labelled cards. Cards with both red and blue labels.
I know this can be done with a search but I want to filter. (Ideally in both the web app and the phone app - if it can be done!).
Is this possible?