when they turn dark it usually means that you have overfed them with nutrients.. do you know how much you have fed them (preferably by testing with an EC meter).. if not with an EC meter, than can you explain how much of what product you used? It doesn't seem at this point that she has any tip-burn (which would be a sign of overfertilization).
I would let her dry out a bit and wait with the next watering, and then just use PH'd water without any nutrients. (If you have an EC meter i would recommend to measure the EC of the run-off).
happy growing!