What happens when you dont need black essence anymore?
Solution 1:
If you have really run out of the need for Darkness, then I must commend you as you've gathered at least the 340,000 required for the Resource Director.
There is also at least one quest (having to do with Nyx, obviously) that needs a few thousand darkness to complete at the house contractor.
Also, as @JMac mentioned in the comments, don't forget that there are 2 sets of mirror upgrades.