I'd go a step further:

* Remove the mana cost from Darkness skills entirely.
* Give them a CD instead.
* Make them stronger.
* Make TBN not have a CD in itself.
* On using TBN, you give yourself a 15s debuff that prevents using TBN again.
* TBN then auto-combos into a button that removes this debuff, with a scaled manacost based on the remaining duration on it.
* As now, depleting TBN gives a free Darkness use, in this case it instantly resets the CD.

The idea would then be that our mana is entirely for TBN, but depending on income we can go wild chaining TBN usage.