All I can add is that I have seen many caste iron props in my time, all on tugs from steam to diesel. They were always black in colour the natural colour when submerged in salt water, plus I have seen a few large 4 bladed props in Naval Stores put out for disposal and they were black. I suspect that in real life if you painted propeller's at the next docking the paint would be worn off as saltwater is abrasive. The big props I suspect were spares for the old Empire class, since long gone, but naval stores have al kinds of obsolete stores tucked away in corners and forgotten about. Remember the SALMO in 1980 ordered a certain size shackle, but was told there would be a delay in sending it, when he asked why the storeman said it had been in store since 1929 and that arrived black.