Ink Sacs obtained by trading won't make black dye, book & quill, or stack with ones from squids.

Only Ink Sacs you obtain by directly slaying a Squid will turn to black dye - Ink Sacs obtained by trading with a Wandering Trader will not turn to black dye, nor will they work in crafting book & quill

Also, the Ink purchased will not stack with the Ink directly obtained by slaying a Squid, nor obtained by fishing - see in picture below, none of these will stack with each other

To replicate, purchase Ink Sacs from a Wandering Trader and try to convert it to black dye and it will not - see images below

Note: This is also the case with Gunpowder