Black leaves on new growth

Does anyone have any idea what this is?

I have a mix of autoflower strains (white widow, ak47 and bubble gum). Outdoor grow. About 20% of new growth (bud sites?) shows black in the leaves. It does not rub off, it’s part of the leaves.

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Here it is again. What’s up at the lab? To many to be a coincidence i think not.