Answer: A purplish-black coloring on solanum leaves, especially if it is concentrated on the outer rather than the inner leaves, is almost always caused by cold weather. There's nothing you can do about it; subsequent leaves will develop normally. If the odd coloration is throughout the plant and on all the leaves, it can indicate a nutrient deficiency. If this is the case, feed with a balanced, all-purpose garden fertilizer, in amounts and with the frequency listed on the label.
Best wishes with your landscape!
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