Honestly, updating TBN alone would make DRK feel fresh again. But I agree, Dark Mind cannot just be magic mitigation the same way PLD can't just block physical damage with their shield. If shields can block any damage then Dark Mind should be able to mitigate any damage. Alternatively, you could make Dark Mind like Excog, if it doesn't block any magical damage then expires perhaps it makes your next TBN block X% more damage on the next use.