Sometimes cold blue will darken those marks, more often it won't (the case-hardened finish seems to vary in its reception to cold bluing). Might be worth a try to make the mark less noticeable if shimming with that aforementioned washer has now established some clearance.
Or a new hammer could be tried, they aren't too expensive, but of course you would again have to have clearance so the marks don't just return. Sometimes just swapping in a new hammer/trigger eliminates the marking, but this is hit or miss and I really don't know why it sometimes occurs.
Pre-war and early post-war S&W revolvers seem to have this problem much less; the tolerances were better, more attention was seemingly paid to "centering" the hammer and trigger re those frame and sideplate bosses.