Magnesium deficiency?

It does indeed seem like a Magnesium (Mg) deficiency! It may be caused by a couple of reasons, one is a calcium toxicity (which is recognized by dark foliage and blotching in mid to top leaves) this inhibits Magnesium, Potassium and other micro/trace-nutrients - this is less likely but keep an eye for any signs of more deficiencies if you cant accurately identify it as Calcium toxicity. Second reason is just lack of Magnesium in your fertilizing solution, most magnesium additives/fertilizers com paired with Calcium or included in the macro-nutrient base fertilizers. First try checking the pH for the correct range (5.5-6.5), then increase or add magnesium in to your solution, and if the problem persists or even shows more signs of deficiency then remove all Calcium fertilizers you use (don't stop with the base/NPK fertilizers) and give her a heavy irrigation to "flush" out the excess Calcium. Hope this helps! Good luck and happy growing :)