Blood of Zul'jin drops from Zul'jin in Zul'Aman. It starts Blood of the Warlord, which rewards 10 (and a bit of personal amusement). There is only one drop per kill.
Rare to Epic
When Zul'Aman opened, the Blood of Zul'jin was a rare drop; in Patch 2.3.2, it became an epic drop, much like the other "parts of bosses" (Kael'thas' Verdant Sphere, Magtheridon's Head, etc.)
Patches and hotfixes
Patch 2.3.0 (13-Nov-2007): Added