The 4th Standard Catalog of Smith & Wesson (SCSW), Pg. 354, says the Model 439 has: "...blue or nickel finish..."
The one (many?) you saw evidently has been, um, *******ized. It's a relatively easy thing to do with the first through third generations of S&W semi-automatics inasmuch as the slides and frame rails pretty well match up from one firearm to the next, even to the point of mixing a relatively shorter slide to a long-railed frame.
Should one not be interested in getting duped, the relatively cheap (~$30) and newly published 4th edition of the SCSW is money well spent.
Later.