Philip, This is certainly a challenge!!! I searched forever to find some, and I ended up making my own small velvet curtains to block the incoming light. After about two days of incoming day light through the side of the curtains I decided to go extreme. I took the the windows out tinted them, spray painted them black, and then reinstalled them and hung the curtains back up. Problem solved!!! total light control!!! Well, sorry to ramble on. Good luck on your quest.
I made the 2.35:1 projection screen in my basement HT out of blackout cloth. My wife used the excess cloth to line the curtains -- a novel use for this product!
Sorry! I wasn't too clear.... they don't sell them off the shelf in the store you have to order them take about a week.
Btw - On a sunny day the only place you see light coming through is just a speck at the edge. There is no light coming through were the slats touch each other, and for what it's worth my blinds are black. You can get them in other colors but my curtains are crimson velore and trying to match maroon color is like trying to teach a 6 yr old how to water ski...LOL