From what I've seen, the shields that are equipable by BLMs are more of a novelty than anything practical. There are also a couple of swords that a BLM can equip, but again just a novelty.
The problem with both of these choices is you don't have a shield skill or a sword skill, so they're are essentially worth it. Having a shield is not so much about the additional defense but is more about being able to block with your shield, which negates an entire attack.
My advice, forget the shield and go with a nice wand or staff that gives you bonuses to INT and/or MP, and never choose one based on damage, bonuses are your friend.
EDIT: Just thought I would point out that I AM NOT a black mage, so take any of my advice knowing that I speak from a Paladin's perspective on what I THINK about Black Mages.
Edited, Thu Aug 19 12:37:44 2004 by Eruntalon
____________________________Eruntalon / Fenrir