You may be referring to little cinders of brake pad material that get thrown off when your brakes are hot. Basically, a chunk of brake dust.
If so, there is nothing that will wash them off. You have to remove them with a scraper. I use a nylon putty knife, which is stiff enough to do the job, but won't dig into the wheel or paint. Soak with wheel cleaner, then gently scrape where needed, then use wheel cleaner to finish up any residue.