I'd not over-clean the blade. Black rust is 'good' stable iron oxide, Fe3O4, and adds character & provenance. Red rust, Fe2O3, is the cancerous version that should be cleaned off. If any Black is loose, lightly sand or brush it off, in any case neutralize it with a baking soda solution after a vinegar etch. Then use a good oil on it, like Ballistol.
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