Hi Roman. I had a look at another version of the colour pic you posted, in the Steve Birdsall book "B-17 in Colour".
Judging by the slight difference in contrast, on my example, between the rudder and the star and bar, I'd say the rudder was black.
The original pic was most likely a Kodachrome transparency, and if a colour shift due to age, storage, reproduction etc was apparent, the blue areas would possibly exhibit a slight red/brown tinge, and black areas a slight blue tinge.
On my monitor screen, the example of the pic I have shows blue for the star and bar, albeit rather dark, and a distinct darker tone for the rudder, without any colour tinge, suggesting black, like the numbers and letters on the fin.