By default all ship radio contacts are recorded as black on your navigation map. For me it comes down to where I am to decide if its neutral or not (for example, being right off Bergen when Norway is still neutral represents a high chance of encountering neutral Norwegian shipping). I still check out a contact if its possible to catch them. I almost let one go in my last patrol when I was assigned to patrol off Kristansand two weeks before the invasion of Norway. What I thought to be a neutral ship turned out to be a Polish tramp steamer
. Word to the wise, always check them out.