GM just used what they called "Chassis Black". It's more of a satin than a semigloss and is easily duplicated. In fact, the best approximation that many people go with is the Ditzler DP90 Epoxy Primer. It has no UV inhibitors, so it can't be overly exposed to sunlight, but for the frame/chassis/suspension, it wouldn't be exposed anyway.
I just finished a frame off restoration on a '72 Chevy C-10 and I decided to powder coat the frame and suspension, instead of paint. The powder coater had a nice satin black that looks very much like the original chassis black, but of course holds up much better.
When the only tool in your toolbox is a hammer, you tend to see every problem as a nail.