Being as Ian and Eric are present day, wouldn't cinder blocks have been the product of choice for blocking windows post 1975? Gives a nice color contrast as well. In that case, HO brick might be suitable.
For me, no. New England mills typicaly built in the 1800s would be brick blanked out in current modern day. Block would actually be a rare 3rd, coming after painted plywood blanks, (in common colors of green, red, or tan)
What also would maybe be nice for someone to do maybe in brass (Bob?) Would be modern windows for those mills that have been turned into condos or office buildings, much like in Lawrence and Lowell, MA, where you have 2-4 IGU panes in alum frames.
