IIRC, it was a blue/black color...I think Aeromaster made it....not sure if anybody else has decided to make it.

Gunze has it in their Mr Color line.

You can also mix dark sea blue and black. 

Yeah, as Nathan says it's easy enough to mix your own. 

I read an article recently that claimed that only the Mitsubishi Zeros had the blue-black cowls, the Nakajima produced versions were just semi-gloss black. 

I use the Mr color version. When dry it is for the most part black. But when it's wet it does have a blue hue to it as light moves over the surface. But like I said, once dry its very hard to tell it has any blue in it.

The model I am making is Hasegawa 1/72 Zero and in the markings of Saburo Sakai. Would that be a Mitsubishi or Nakajima plane. 

From poking around on the web I'm guessing a Mitsubishi aircraft though I'm not totally sure. With the green over grey scheme it's easy to tell, the Nakajima aircraft had a demarcation line that swooped up from the back of the wing to the tail plane, the Mitsubishi aircraft the line was straight. With the all grey/ame-iro aircraft it's harder to tell. 

Zero Camouflage Schemes by James F. Lansdale  (Web view)