Depends what paint was used to black out the windows, white spirit wont do you much good if the paint is water based, which it might be, could be black masonary paint.
Try hot water on a sponge, let it soak a while and see if that loosens it off.
If its alkyd based resin then maybe a poultice may help but try the hot water first, if that doesnt work then ill tell you how to mix up a poultice.
Mark