In older comics blue was a highlight color for black and sometimes artists used enough shadowing to make it work. (Superman & Lois's hair for instance)
Although in this story we seem to have very little black used on the Black Hood and the policeman uniforms are the same blue.
I know they only had four colors, but they could have made the policemen uniforms a lighter blue.
There's an old Batman story that describes his uniform as black, not blue and gray... of course they also describe Robin's uniform as black as well, which is quite the stretch.