You can buy paint from SEM called "Trim Black 39141". I worked in Chevrolet parts for a few years, and to me it looks exactly like the GM black.

Like the old GM parts black, it is watertight, requires no primer, and is a semigloss black. It is also for exterior use, so it won't fade.

I have used it for decades for bumper brackets, hinges, frames, inside fenders, or whatever else you want. This is lacquer base, but I think it is also available in enamel base. It is a real "cure-all", so I always keep it in stock.

Check with your local paint stores. One of them probably carries it.