Anthocyanin deposits, kind of like a person getting zits. I see em once in a while, looks like ink drops fell on a leaf. Nothing to worry about. If you also get leaves getting blotchy purple it's a sign it's struggling with phosphorus load but that only tells you there's a mobility problem, not that it's got too much or too little or roots are just too cold. But just having black freckles, don't sweat it. You might notice they tend to be localized, not all over the whole plant. That more than anything is what makes em seem like ink spots.