Act 2 Darkness

Entering the Waterfall Cave Level 1 and coming back out to Pools and Streams will eliminate the "Darkness" in that area.

EDIT:
Clarification: It's still 'Dark' in a sense when comparing the map to the state it's in when the boss has been killed, but it does not have the dark purple colour filter and there are no instances of the black stream flying around when there should be
Screenshot