Blackmages are actually rare. I've only come across 3 or 4 thaums so far in dungeons, and only 1 was past lvl 30. Their sleep and hard cc is a godsend for parties though, especially for a tank who can't handle mobs or is undergeared. I really think they're an underappreciated class.

BTW, I know there is some demand for BLMs in FCs on the Leviathan server. It's not as high as the demand for healers or tanks, but it ranks pretty close.