How should I treat window behind listening chair?

I have drapes on 3 of the four walls of my listening room because it's a very small room and had annoying slap echo (clap your hands and hear it ring). I find the drapes hugely improve the sound.

However if your system sounds dull and lacking in treble detail the drapes will make it more so, as they affect high frequencies proportionally more.

Blackout has less, considerably less absorption coefficient due to the black out foil, however, the foil does create some diffusion. So it's not really a one is better vs the other, it depends on the room and placement.

Marigo Labs makes VTS tuning dots to attach in proximity to corners of windows, as well as other types for every part of a system, and room.
Can check the website, marigoaudio.com
Also, can email Ron Hedrich at the website. He creates the Marigo Labs products.

Brick it up!
Seriously: I think it's nonsense to hunt for the "ideal room" because there is no such thing. In every concert hall there are hundreds if not thousands of "compromize-spots" to sit in. Who would like to sit in the same seat for a - let's say- guitar recital and an orchestra concert, for a William Byrd mass and Orffs Carmina Burana, for a jazz big band and a choral concert?? I doubt that your window - while probably slightly changing the sound in your room - is doing any bad effect to your LISTENING TO MUSIC which means that any damping will only CHANGE the sound but won't improve your enjoyment of the music!
Relax and enjoy - or call in the brickie.

I put a comforter behind my speakers over the window and it made a huge improvement. Don't worry about absorbing some sound. It is necessary to get clear imaging and kill brightness.