User blog comment:LOTRMod/NPC Health/@comment-25604636-20160513125947/@comment-25101690-20160513164001

Actually, the term "uruks" (short for "uruk-hai") should only be used for the general race, a tall breed of orcs (despite the word actually meaning just "orc folk"). Therefore, Gundabad uruks are just especially big and strong Gundabad orcs, but not necessarily experienced. I think their mention in the LOTR books states that they were probably sent from Mordor.

Of course not all uruks in Mordor are Black Uruks. You can't have elite troops without regular troops, because elites have that title to compare them to regulars and are usually recruited from well performing regulars.