Don S, look into that PPG industrial line I mentioned. It's alkyd enamel. You could apply a compatible black 1st, then use your old paint for the final coat to avoid a mismatch if that's a concern.
It may seem funny to mention mismatch about black. Trust me, there's a lot of different blacks. Some are brown-tone, some are blue, some grey. Take a small amount and thin it down like 200%, then rub it on chunk of white paper or cardboard. Where it's transparent you'll note the base used. Lacquers and old enamels from Ditzler were usually brown-black, DuPont lacquer used to be blue-black, urethanes can go grey or brown depending on the formula. Just sayin...