You won't be able to get JP up without serious grinding and never job experimenting till at least CH4 (roughly half way into the game) because there is a tremendous JP requirement jump when you hit 9 specifically you go from needed 2500 to needing 6000. Grinding really isn't worth it just play the game normally and experiment with the jobs.
Though to be honest Black mage is kind of terrible it's at least worth getting to lv6 normally and once you hit ch5 and everything get a huge difficulty spike JP gains will spike as well and that's when it's best to grind lesser jobs for their abilities. Black mage has two more worth while skills at job lv12 and lv14. But otherwise black mage is going to be outclassed as soon as you get spellfencer in just the next chapter.
A white mage and Monk on the other hand remains useful for the entire game though you still shouldn't fall into the trap of being in a job to long. In general it's just better to level a job up to 9 and then switch. Then once you beat Ch4 or that's when it's best to start grinding jobs up past 9.
I've got a quote that embodies you perfectly, but it's seventy-three posts long, has a few massive flowcharts, and lots of Xion-KMA to Me