Had a go, Ruth, but I think you might be right about about returning to the microfilm images to be able to recover the blackest parts, if you can adjust the lighting while viewing them.
What I've done is, first, changed the mode from greyscale to RGB colour which can sometimes help when adjusting the lighting; then adjusted the lighting, specifically lightening the shadows and reducing the contrast. The trouble is, in the blackest of the black parts there is just no depth, so nothing hidden in the murk.
I've attached my try. Others may do better or have better advice.
Peter