Can the purify food and water spell fix water impacted/poisoned by the presence of a black dragon?
I assume the power of the black dragon would overpower the spell, but I'm not totally sure.
The description of the effects of a black dragon on the area specifies
Water sources within 1 mile of the lair are supernaturally fouled. Enemies of the dragon that drink such water regurgitate it within minutes.
Not all water is fouled, just the sources of water. Therefore drinkable water can certainly exist in the radius of a black dragon. In the absence of a rule specifying otherwise, purify food and drink should work as normal and purify any water it is cast on.