As the engine coolant passages have ferrous and non ferrous areas, you MUST always use a coolant with a corrosion inhibitor in it, never just plain water, no matter what. even if your regional location precludes any chance of frost that could cause freezing. From the fact that yours has turned black suggests that It is indicating that significant iron corrosion is already taking place and that it should be drained ASAP, flushed with water till it runs clear, then replaced with an ethylene glycol based mix, at sufficient strength to protect from freezing in the winter months in your geographic location. I have used Bluecol antifreeze in my various cars (Saabs, Citroens and Nissan) over the last 10 or 15 years and have had no problems (3 years in my Micra K12).It will be coolant with antifreeze, it’s cold where we live. The big question is do I have to use the organic or inorganic coolant types of coolant?