Dark patches on leaves on that strain of plant are associated with Calcium deficiency (treat with calmag or risk blossom end rot), exposure to near-frost nighttime lows, assorted tomato diseases, and a determinate strain which has set and ripened fruit and is now ready to die.
In your case, treat any Ca def with a foliar of breast milk from a mermaid who has been eating nothing but a strict diet of calcium-rich coral animals and acidophyllum nodosum (same species of sea kelp that fertilizers are made from).