Azusnek is based off a black mamba, and as such the inside of their mouth is a jet black color, like in this pic.

Tho, Azu *does* have a slight purplish tinge...

also, Azu's venom is both a potent hallucinogen *and* a painkiller! (the latter part pulled from the fact black mamba venom contains mambalgins, which when isolated appear as powerful an analgesic as morphine without the respiratory distress associated with it)