I had the same issues with black gold soil with my oaxacan plants. For me it was Calcium and magnesium deficiency. I didn’t add any lime and I should have. You’ll soon have nitrogen deficiency too if your plants do as mine did. It’s really a pretty poor soil , nice structure but not enough nutrients. I Watered with 20 drops of cal-mag per gallon every watering, and top dressed with some seafood compost to add calcium and magnesium, and they are slowly looking better.
It could be something else of course but i just went through this with black gold, though i had potting soil…good luck.
Yup.