How do you get black stains off wood floors?

Rub plain white toothpaste with a soft white cloth.

Do you know what created the stain? That may help you to find a more effective solution

Neutral shoe polish works well.

If the black stains were caused by pet urine there is nothing that will take the stain completely out...(owners before me were not good pet owners)

I have bleached the wood and re-stained but it still shows compared to the surrounding wood.

If the stains are from shoe heels, a tennis ball on a stick rubbed dry on the mark takes it off.

If the finish is worn and the wood is oak it could be water stains. Oak does that. If it's shoe msrks try vegetable oil. Then clean it.