What do you need, blackout blinds or blackout curtains? You can buy blackout lining which you can add on to the back of a curtain. You may not even need to buy one for each curtain as I used to do it on just one and pull that curtain more than half way over the window in the summer (it was quite wide so covered more than half the window if needed) and keep it as normal in the winter. Hth.