As Kinnison already answered, you have to have both classes on same character. All jobs have two subclasses from which you can pick your additional abilities for the job. That's why it's a good idea to also level both subclasses as early as possible for the important abilities. For BLM your subclasses are Archer (needed to unlock the job) and Arcanist.

You will want at least Raging Strikes (level 4) and Quelling Strikes (level 34) from Archer. From Arcanist Virus (level 12) and Eye for an Eye (level 34) can be useful for your party.