Let me tell you how I blackened my media room, as seen here.
1. I did paint the room black.
2. I took cardboard from boxes and placed them in the windows. I then used black tape around their perimeter.
3. I then go good looking, but relatively inexpensive black curtains
I actually did this temporarily util I got either a more expensive solution or a more technical one. But it works perfectly and was very affordable.
The second picture is very old and does not have more recent equipment or layout, but shows the window.