The black powder is a common fungus called *Aspergillus niger*, where *niger* just means black and has nothing to do with Niger, the nation named for the Niger River (*"gher n gheren"* meaning "river of rivers")
Wash the onion, peel off the papery skin and/or outer layer, rinse **t**he (hopefully firm) onion again under not-hot water, then wash hands.
Toss the whole onion if the fungus has spread to the inner flesh, if it smells bad, or has soft, slimy spots. #finance