They could have used matte black tempera (Sanford's IIRC). Back in the day, I played around with (analog) photocopier rendition of pen-and-ink drawings and foun

They could have used matte black tempera (Sanford's IIRC). Back in the day, I played around with (analog) photocopier rendition of pen-and-ink drawings and found that large black inked areas developed faded centers. Coating the ink there with said tempera fixed the dropouts.