From what I've learned if it's an express blue then the parts need to be hot while the solution is applied. If it's a slow blue then the parts are at room temperature. Most of the commercial solutions are express blue. From what I gather if you swab slow blue on a hot part then the rusting action is too vigorous and the finish will be matte.

The PH of the water is something I read in a really old text and it was just a final snippet. "The worker should be aware that the softness of the boil will affect the final color. This can be adjusted with a dropper of HCL".......and that all there was. I figured out the colors by experimenting.

Another trick that can save you several rounds is rub the first application on with steel wool if your using an express solution.