I mean, assuming the enchantment is what also makes all the monsters human-compatible, I would figure it would remove the cyanide in her body. Not much point in enabling wild monkey sex if all your sexual fluids are full of toxic chemicals. Even after it wears down, Slimes are shown to have their appearance change based on food preferences - NOT the makeup of their bodies. Besides, cyanide isn't red, so it's far more likely she's taken pigmentation in, and expelled the toxic byproduct after the fact.
Also, just so you know, this wouldn't even be an issue unless someone ATE Perpetua. Cherry pits DO NOT have cyanide in and of itself - they have a compound called amygdalin that WHEN METABOLIZED BY HUMANS breaks down into minute quantities of cyanide. Moreover, the amount of cyanide needed to prove lethal is equal to approximately 1.5 times the numerical weight of that person in milligrams. So, to kill a 150-pound adult human, you would need around 100 grams of cyanide. The average amount of cyanide in a single cherry pit is about .17 grams, meaning almost 600 would need to not only be metabolized but also NOT DISSIPATE. In order to reach the concentrations you're thinking of without dissipation, Perpetua would need to eat literal tons of cherries each day.
So, it's not really an issue.
Source: My father, who has worked as a chemist for almost 50 years. He gave me some really odd looks at my questions.