Yuta has literally healed and "purified" Naoya from Choso's poison which functions from his very blood cells.
He can do it
None of which are actually poisonous because of chemical compositions, they are poisonous because of Choso's heritage from his ties to the Death Painting Womb, which are Cursed Spirits with Cursed Energy - so his blood is poisonous primarily because of the Cursed Spirit heritage that comes with the Cursed Energy.
And even if we assume the purification works at the cellular level and can work on chemical poisons that
aren't based on Cursed Energy (which, if I recall correctly, no Sorcerer actually has any scans or statements of using their abilities to cure chemical poisons that aren't based on Cursed Energy), that wouldn't matter because the Blacklight Virus works at the
molecular level - and it has layered Biological Manipulation and Disease Manipulation hax that works even on victims that have resistance to molecular-level Biological/Disease Manipulation as well... That also includes the Blacklight Virus being gaining adaptations that allows it to resist the Parasite (which the Parasite was able to infect an entity completely made of nothing but viruses and virions, such as Mercer himself) and even being able to infect Whitelight itself (and Whitelight is meant to be the cure to the Blacklight Virus... Yet, it got infected anyways).
Molecular-level hax > Cellular-level hax every time. There's also the facts that the Blacklight Virus aren't poisons, they're diseases (specifically, viruses), so they also work fundamentally differently from poisons. Curing poisons isn't necessarily the same as curing diseases after all.
So, nothing says Yuta can try to purify the Blacklight Virus and be shocked that it doesn't work due to the Blacklight Virus resisting its attempt to cure it.
And that doesn't even take into consideration that the Blacklight Virus itself has Reactive Evolution - so, even if somehow the purification can inconvenience Mercer, nothing says the Blacklight Virus can't eventually just adapt (which, can take up to seconds or even over a minute based on the feat of the Hunter with compatible DNA resisting the Parasite feat, which Mercer absorbed and gained the Reactive Evolution abilities of) and have Yuta's Purification attempts become ineffective by the end of it.