The Shadow Ink is a component added by Forbidden Magic. It is a dye that can be crafted from Umbral Roses and serves as an alternative to farming Squids to refill Scribing Tools.
There's no sight more pathetic than a soggy Thaumaturge. For all your great and miraculous powers, you still have to go jump in the nearest lake and hunt for squids every time you want to refill your inkwell.
You're done squid hunting. By infusing a rose with tenebrae and victus, you create a flower that spreads like a weed and whose petals can be crushed for ink.