Hi thelewisgang,
I am a bit of a do-it-yourselfer and have tried to visualise your set-up.
If I understand correctly, the end slats of your blinds are each 3ins off the wall surface.
You mentioned a vertical runner which I feel sure was intended to read as the horizontal strip from which the slats hang and which is fixed to 4inch brackets to allow the blinds to be vertically tilted or drawn open.
To be absolutely honest, I know very little about vertical blinds but was given a demonstration of how they work by neighbour friends who have now moved to live near their daughter in Australia. Consequently, I can't , at this stage, further examine the mechanism involved with the blinds, which could be similar to yours.
Nevertheless, I would think that it should be possible for an additional slat to be positioned and angled each end of your blind (un-coupled from the chain linkage) in order to fill the 3inch gap which is causing a problem.
It's just a thought and I shall not take offence if you tell me that I am being stupid.
Best wishes ........Ron.