The single best way to restore the colour and sheen of black Delrin on Simplex derailleurs, Stronglight Hinault headsets and everything else made in this wonderful material is: OIL. This is why Delrin was used (and still is) used for engine mountings etc., it's actually nourished not degraded by oil. You can take the whitest, dullest Simplex derailleur, dab a nice quantity of high quality oil (I use Sturmey-Archer oil) on it, let it soak in for an hour or so, buff it off and presto! it instantly returns to dense, semi-glossy black that lasts and lasts.
Oil will also nourish and protect white Delrin but I've yet to discover a miracle method of whitening it.
Washington DC USA