What is the black? Your abrasive, your lube, or residual metal that has been polished off?
There are two choices, solvents or alkaline.
If this is small and you can do it safe, you might try the strongest solvent that you have available. I always like MEK.
The more enviro friendly method is hot alkaline. Use a comercial cleaner, mix per instructions, apply as hot as possilbe and rinse with pure water to prevent spoting.
Often when items are being polished they are acid cleaned before the final polish. This helps remove residuals that would mess up the finish. In some cases dilute phosphoric acid is used as the lube/coolant during the polishing.
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Rust never sleeps
Neither should your protection