Please Do not use cement away , this is an exterior only product -hydrochloric acid will cause you much more problems than grey black grout. I would stay away from anything acidic, as This can also whiten the resin in an aglomerate tile. You could test a light alkaline product , we have power clean ( others makes avalible ) and a white pad , as Julian , you will be lucky to get this back. Have you had a test scrape, is the black colour under the surface, if not - cleaning won't help here.
Could you use a grout revival product on it, other than rake it out
Could you use a grout revival product on it, other than rake it out