Black iron? That sounds like hot rolled steel that still has the scale on it. The scale is just iron oxide that forms when oxygen binds with the surface of the hot metal. Sheet metal that has a shiny finish as been cold rolled. I don't recall if the scale is removed mechanically during cold rolling or if it is removed chemically (pickling.) But I digress.
I would put one of those flap grinder/sanders on my angle grinder and use it to remove the scale from the cooking surface. Then season it as you would cast iron. Or just season the scale if it looks like it is solid - e.g. not flaking off. If it comes off on the food it is more a cosmetic than health issue.