How can I prevent light from coming in around the edges of black out blinds?
I have a bit of a botched solution for this that I use in one of our bedrooms. The curtain has a blackout lining but sits about 8cm away from the wall. Light bounces off the lining, onto the wall and into the room. 

I made a strip of black plastic by taping together pieces, the strip is about 5 metres long. I drape the strip over the curtain rail brackets such that the sides hang down and block the gap around the sides of the curtains. This eliminates about 90% of the residual light.

I'll add a photo later, since my description is probably not very good.