Black paper will work. For variety you could randomly paint the back side of a few windows with dullcoat which will give the look of frosted glass. Small sheets of brick styrene in place of a window here and there shows some changes to the building over the years. Blocked windows make the most sense on a ground floor. City Classics has sheets of colored curtains and drapes, not quite correct for a commercial type building but I'm just tossing out a few ideas.
Lobuc Valley RR
"The Hobo Route"