Use
Ink Sacs are a vanilla item. They can be used for dyes or to craft the Quaesitum.
Obtaining One
They can be obtained by killing squid. In yogbox however, the squid's spawn rate is low and only spawn at night. Follow these instructions: escape>Global Mod Settings>Mo' Creatures>Water Mobs>Squid Spawn Rate (set to a reasonably high number).
Crafting
Ink Sacs are only used for the Quaesitum and dying wools.