Reworking Dark Arts probably would be the best option and make TBN require Dark Arts to use and remove the mana cost.

For example: After taking 50% of your health in damage (damage while shielded still counts), you gain Dark Arts. TBN would be self use only, with no cooldown, just dependant on Dark Arts.

This way if TBN is fully consumed, you would have met half the threshold for another TBN already.

Add a new ability that is cast on others.
Dark Heart: Apply a 20% health shield on a target, if this shield is consumed the Dark Knight generates Dark Arts. 20s CD.