In my opinion police uniforms, especially Met Police uniforms should look closer to black than blue. I paint my police figures black and then highlight them with a very dark grey/black mix.
Talking from experience the uniform I wore when I served in the Met was so dark blue it looked black. I know it's not relevant to the 70's but in the late 90's we were issued with pullovers that clearly were blue but the contrast with the rest of the uniform which was made from the same material and pattern used for decades was clear to see.