I found some blackout material made by Rockland Industries that is sold thru drapery stores. It is thin, flexible, and a fraction of the cost of other material I have seen offered for photographic use. It is white on each side.

I haven't made anything with it yet but I wrapped a piece around a 500W quartz-halogen light fixture and it totally blacked out the room!

I'll be trying it for a bellows soon and will use it to replace my black blackout cloth (which is HOT in the sun!). I'm also going to replace the fixed shutters in my darkroom with removeable cloth curtains from this material.

http://www.roc-lon.com/Blackout_FAQ.htm