I've used the Birchwood Casey brass black on my model ships. I like it a lot better for blackening brass than blacken it. I wouldn't use either for priming. They are both very finicky for blackening. The Birchwood Casey seems to be less so, and does a little better when the parts have been silver soldered. The parts need to be really clean (I usually soak in a mild acid, then acetone, or put in an ultrasonic cleaner) first. Diluting the agent helps prevent the thick crusting that Nigel wrote about. I've also started using copper when I need blackened metal. the blackening agents (different from what you are using) for copper don't stain wood, so sometimes you can install the part, then blacken it. A lot of times, paint is a lot easier.
