If it was a conjured animal the blood would remain until the animal unmade itself. [ Hallucinations are something Severus can buy, sure, but it's too simple of an explanation by itself. ]

It's not alive. [ He's perfectly happy to be seen as brilliant, but he doesn't know if William understands the skill required to do what he does overall, or if it's just a reaction to the 'oo shiny' aspect of magic. ] This is transfiguration. Eventually it'll want to be a glass jar again.