Vinyl(the other kind)

@Schubert- I'll take a stab though I'm not an acoustician. It would seem to me that vertical blinds would break up the sound, and you could vary the effect by how they are angled, but the backdrop still being glass would remain an issue. 
I think people have used old shutters as an adjustable diffuser for certain things-
Why don't you experiment and see what angling the slats does? Fully closed, they would obviously be a reflective surface. I don't know if lining them on the side facing out would help--would seem to be some labor for questionable results. Interested to see what others think.
best,

My whole listening room is majority glass panels.
I have vinyl blinds on all walls, trial and error has proven that approx 3/ 4 shut has given best results.
Still a shite listening room though....

Thanks fellas, about same thing I've found , just tried of nearfield listening .

Yes, but not much

I asked ASC this same question recently 

you might also put a fake ficus bush/tree behind your speakers

Actually they made a huge difference in my room. Without any blinds it was truly horrible, no focus, no snap, bass was booming. Three quarter shut angled up truly worked wonders.
Does not help having a tiled floor either but large rugs and 3 large squishy sofas helped there as well

I have exactly that behind my Ohm F5s.

Very practical.  Definitely makes a difference in my case and very adjustable. YMMV case by case.