The Stowa website shop usefully explains the option of blued screws, including the following:
"By the way, we use natural steel - you see the result will have no silver screwslit. If you see silver screwslit your blued screw is a screw which was before nicleplated. After this plating the screw was polished - but it is not possible to polish "inside" the screwslit. This is the reason that some blued screws have a silver screwslit. If you have ordered your watch with an chronometermovement you don´t have to order blue screws extra. The chronometermovements include blued screws."I have examined the chronometer movement on my beloved Antea and find that the blued screws indeed have silver screwslits. This does not bother me at all! I suppose the screws were part of the movement as supplied by ETA and not the Stowa-finished screws. But I thought I should log it here, where everyone is a Stowa admirer and will accept it in the spirit in which it was contributed.
[awaiting my Stowa Yellow Chronometer, sitting in the Karlsruhe FedEx depot]