Job Combinations
Black Mage/White Mage-This is the best subjob for blm. With the a whm sub you will gain almost all the very best whm spells. By the time you reach lvl 75 blm, you will have Cure 1-3, Raise, Reraise, Teleport (Holla, Dem, Mea), Invisibility and Sneak. With a whm sub you truly become the total mage package.
Summoner- Some People believe that Sumoner is the way to go for the job trait Refresh at 1mp/sec. The flaw in this thinking is that you are trading off all of those whm spells for weak pets and a1mp/sec. With out the Summoner two-hr you one of the most critical of there abilities.
Thief- I only mention this Subjob because it get over looked by so many. Although it is not a practical for most situations, it is very useful for farming after level 30 because you gain the job trait Treasure Hunter. In most cases it makes a dramatic difference in drops, with one notable exception, the Papyrus Paper for G1. Not sure why, but it seems the thf sub does not help much in that situation.
Black Mage as a Subjob
Black mage make a good sub for almost any job below level 10 because you get Stone at level 1 and with a pie you will have ample mp to kick but. Low to mid-level Whm�s and Rdm�s will want to sub thf blm but in the higher lvls blm does not offer much to any job. The nukes will gain will not do much damage because of your meager Int vs. the mobs Int.