The black stains are typically spots where rust was once active. You can soak in 50/50 white vinegar/water 30 minutes and scrub with Chore Boy stainless steel scrubber, steel wool or medium stiff SS wire brush. Or do as I do and spray straight white vinegar directly on the spots and scrub with a Chore boy/steel wool/SS brush. Alternatively, you can leave them and they will blend in with time/seasoning/use, but obviously not as quickly as on the cooking surface of a pan. Would not use brass on cast iron, it can transfer and be something else to work to remove.