So looking at the potency per mana cost of all our skills it seems like the optimal way to spend mana with these numbers is using it entirely on Blackest Night >>> Non DA Bloodspiller >>>> Blackest Night >>> Non DA Bloodspiller.... repeat since BN/NonDA BS is .158 potency per mana which is the highest of all our skills and only on a 15 sec cooldown unless I am missing something.
Now It looks like BS is on GCD so if you take the average potency of a gcd for normal SE combo and add it to potency of DA/CS it looks like DA/CS will actually have more potency per mana by a bit with the gcd cost factored in but seems to me BS is still better than all other uses.

Someone tell me why this is wrong please.