[Twi]

The manga recently updated. They explain more on the magical inks, like how additions of certain things prolongs the effect or makes the lines invisible.

Spoiler:

Then it turns out that the Brim Hat replaced Coco's Ink with his own blood-made ink, enhancing her magic so that her spell ran over its normal strength and ended up drying up a river. Qifrey, whose out for blood against them, ends up erasing the memories of the ink-maker to hide that fact so that he can find them.