They will be on either end of the filling loop. They probably won't be black, and they won't look much like taps! Usually they are a quarter turn valve, like a washing machine tap. You may need a screwdriver to turn them, and possibly a pair of pliers if the handle has fallen off.
depending on the boiler make/model you might have an internal filling loop ie worcester need a plastic filling key, post more details and we should be able to help
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