These are like cloth blinds, that pull up the same as blinds but don't open, that is, they have no slats, you can get them in whatever color you line, I gave one example from Target below and you can get them in all sorts of widths. If you go for external mounting then chances are you don't need to get custom width cut ones and external (meaning in the house but outside the window frame) blocks the light better and when not in use they pull up and if you have a valance (sp) short shade then they are hidden when not in use, that is how mine are.
So I have the regular curtains, and they have that short curtain on top, and then the roman shade is mounted behind he short curtain. So you can keep the look you have. If all you have are blinds, then they just mound infront of the blinds and again just pull up out of the way when not in use.
Make sure not to get white or light colored ones, even with the termal backing that will "shine" so pastels are best. You can also buy black-out curtains, they are expensive but do the job also.
On Top of that you can give yourself some "green" points as the thermal ones cut down on your airco and/or heating bill.
My wife is very picky so I needed something hidden, but I find I really only need them when the light is really bright. The other advantate of roman blinds over black-out curtains is they are a LOT cheaper usually, at least at places like Target and Home Depot.
http://www.target.com/dp/B000GUKGEY/186-4918125-7742933