Black Mage Question

[Sagittarian]

SO do I level my current character to thirty and then log off and create an archer to 15 then I can log off and make a black mage or what?

No.

Or do i level Thaumaturge to 30 then change it to archer and level it? I'm confused but not sure about all the jobs and professions like alchemy and goldsmith.

Anytime after getting thaumaturge to 10 you can start visiting other guilds and joining them. You have to complete enough story to get the airship to get to Gridania to pick up Archer though. When you visit a guild and join it, they will give you a starting level weapon. Equip that weapon and you become that class. Equip your thaumaturge weapon and you are back to being your thaumaturge.
Once you join another guild you will also unlock equipment sets, so you can save all your gear including weapon as a set for easy switching.