I did a little research. The ink sac is indeed attached near to the anus. It’s thought to be an evolution of the hypobranchial gland still present in many mollusks, and it may have originally been used to trap and excrete dirt particles when breathing through the gills - I guess very similarly to mammalian mucus in the respiratory system. It’s used in conjunction with jet propulsion. We’re not sure what the use of the melanin that makes ink black was (it’s kept it’s current form in cephalopods for at least 300 million years) but it may have been a waste product.