In my experience, the reason for uneven black is uneven finish
before applying the patina. So, do whatever you can to clean that
metal. Suggestions include:
-wire brush on a flex shaft with soapy water
-wet pumice on a toothbrush or bristle brush on a flex shaft
-the metal prep chemical sold by Reactive Metals
FWIW, I feel that Griffith’s Silver Black, which is now a Grobet
product, works better than Black Max.