there is more than one way to get a black oxide-looking finish, but no, not every bolt on the front end is finished in the black oxide finish. Many are, though. And those that are not, perhaps were bare metal OEM, I might be tempted to apply it there, as well.
I use, and have used, several of the Caswell products over the last... decade or so, and I am quite pleased with their products. Once you learn a given plating process, and become acquainted with the chemicals and solutions used, you may find other cheaper sources for refills.
For a one-off, or for working on one car, a Caswell plating kit may seem pricey and/or overkill. But for repeat offenders, and working other cars, projects, clients, etc., they are among the best ways to get started plating if you live in the US, IMO.
For even smaller quantities, for example, were you to pull ALL the bolts you want plated into one pile at once, it may be cheaper and faster to send them out to a local service provider to plate them for you. They will all generally have "batch pricing" as this is how most B2B get things done.
But the process of restoring, learning what is what, learning the processes, the techniques, and so forth to produce the results you want, are experience that last a lifetime. I actually get a little thrill taking on a new project where I expect to learn something new, something I hope to reuse later in my ever-dwindling lifespan.
