How do i turn regular shades into room darkening ones?

What are your current shades made out of? I haven't tried it but I'm not sure that spraying/dying fabric curtains black would make a huge difference.

If you don't care about how it looks, the cheapest and most effective way to darken your room is to just tape up some large black garbage bags over your window, behind the shades (and if the shades/curtains are big enough, they'll hide the bags).