Tomatoes with black bottoms sounds like blossom end rot, and calcium uptake is a factor. I haven't used the the maxi series nuts, but for continuously flowering plants like tomato you should be using the maxi bloom nuts, otherwise they may not be getting the right mix of elements. Contact general hydroponics at
tech@genhydro.com for the best clarification on using their
nutrients.
But at the same time calcium uptake is related to environmental conditions as well, not just the amount of available calcium in the
nutrient solution. You can have plenty of calcium in the solution, but environmental conditions can affect weather the plant absorbs them or not (and evenly). Especially those that affect transpiration, that's what supplies the water (and the calcium in it) to the plant, and developing fruit. Such as low humidity (or high), drying winds, uneven watering to the roots etc.. In witch case adding more calcium to the nutrient solution won't help.
Here are a few links about blossom end rot
Blossom End Rot of Tomato (scroll down)
Blossom-End Rot of Tomato, Pepper, and Eggplant
BLOSSOM-END ROT ON TOMATO
RPD No. 906 - Blossom-End Rot of Tomato