Home depot has a temporary, paper, accordian type of mini-blind. I used some when I built my house and had not yet put up the blinds.
I haven't tried this yet, but I soon will and will post my thoughts on the results.
Anyway, I'm thinking of getting a few (they run about 5 bucks) and painting them to match my wall color and applying a dot of velcro on the wall and blind to make for easy application and removal.
I'm going to limit their use to my home office and place them directly behind my monitors, which for space reasons are much too close to the wall, and perhaps one on each side of the speakers, though they are not particularly close to the side walls.
Just one of my current musings...