I have left my cast iron pan on the heat for 2 1/2 hrs. How do I stop the release of black on paper towel and when first cooked onions? I left my empty cast iron pan on the stove top for 2 1/2 hrs.  My problem is not a rusty residue, but black that continues to wipe off. What can I do? I have washed with steel wool, rinsed well, but when I wiped with oil, the paper towel continues to wipe black.
Fully clean it
The residue is burnt seasoning (fats) that you haven't fully removed yet. It takes a surprising amount of effort, but this article describes how to completely strip your pan so that you can re-season it.