I used a roll down style blackout shade from Depot that was $60. I wouldn't describe the effect as blackout but more like room darkening. On it's own it is not enough.
The $10/panel walmart blackout curtains do work better. Together they are quite dark, but there is still a ring of light that gets through behind the curtain as it stands off the wall about 3" and you might get some light where the 2 panels meet.
If you want all of that light gone, use both of the above, and then you can get some black sheets, roll them up and hang them around the back side of the curtain.