Ink can drink the ink he spits out (and than bi and not)
His black ink is essentially vomit, which is most likely monster magic or similar. It would be basically consuming your own vomited leftovers if that makes sense. Theoretically possible, but quite gross to do. Since he does drink paint vials, that is what the black ink is.
As for Ink, he at the very least is into shipping. Outside of that, it would be hard for him to feel genuine love due to his senses and soullessness as of now. He was aroace in the original canon. How I view it is as Ink liking the concept of shipping but not being big on having a relationship himself. So no, he may not be bisexual. That is only my perspective though from what I can gather, so take it with a grain of salt.
-Levi-Weaver
