Entomology (Study of Bugs)/Small black pods on windows and window frames
I just discovered a bunch of small black pods or spores all over the glass of two corner windows (scattered, not in a pile), along the frame of the window, as well as a few clinging to the insides of the drapes. They are very tiny, black or dark brown, hard. Almost like a poppy seed. They were very tightly sticking to the window panes so I had to use a nail to scrape them off. Unfortunately I cleaned most of them off before I went online to search for clues as to what they are. However these photos give an idea of what they look like -- there were just a lot more of them. The picture with multiples is up high beyond my reach and gives the best example of the number per area/spread. They were stuck to the window glass as well as the wood frame of the window and the inside of the drapes. There is a large potted fiddle leaf fig in the corner but I see no evidence of a caterpillar. This corner is also the location of the Christmas Tree every year, although it's been gone for a while. However, I didn't look closely at the windows since it has been removed. There is no evidence of these in any other window in the room. (And I know what fly poop looks like -- this is not fly poop). We live in Northern Virginia.
I can't see them clearly enough to be certain but my guess is these are not insects or mites based on your description. Neither insects or mites would be as difficult to remove as you describe. This sounds more like something has dried on the glass. Does the material re-appear after it is removed? When you scrape it off will the debris dissolve in water? Will the spots wash off (as opposed to scrape off)? Post a follow up and I'll try to help you figure it out.