It depends if the marks are on the emulsion side or the reverse side. Rewashing the negs with an after soak in wetting agent would help get rid of dust esp. if you could gently clean the non-emulsion side with a cotton bud or similar. I've successfully cleaned greasy marks and other crap off transparencies with pure (98%) isopropyl alcohol and a cotton bud so maybe this will work with B&W negs.
-------------------- "I wanna hold your gland". Lemming & McCartney