I bought the kit some time ago and even have Peddinghaus decals.

Question: the kit specifies black trousers while the Osprey book on German Police indicates dark blue. Which is correct?

Also, I notice that these are the pre-1938 uniforms. So did they change to the other, or only for dress uniforms, or what?

Can't really address the trouser question fully, but I used to have an extensive helmet collection consisting of Allied/Axis headgear (I sold it to pay for my last year of college in 1975, but that's another story). In my collection I used to have a double-billed steel luft polizei (air raid warden) and Feurwher (fire service) helmets-- they were both black with the Fire service badge decal. It strikes me the service uniform that went with those would have been black as I think about it, because it would have been easier to clean. Not sure about the dress uniform.
VR, Russ