Black Uruks

The Black Uruks or "Mordor Uruk-hai" were an elite breed of Orc that made up part of Sauron's vast, ground-bred army, and thus Black Uruk is the term used to refer to any Uruk of Mordor.

History
Black Uruks were the first Orc-kind of Middle-earth to be named uruk, for they originated before the Great Years, and first appeared in assaults upon Ithilien that took place in 2475 of the Third Age. (It was almost a millennium later that Saruman in Isengard bred the more familiar Orcs specified simply by the name Uruk-hai.)

Boromir spotted Uruks of Mordor in Moria, soon before the skirmish in the Chamber of Mazarbul between the Fellowship of the Ring and the many Orc-kinds inhabiting Moria.

Within Mordor, Frodo and Sam later saw Uruks march from Durthang towards the Black Gate.

Peter Jackson's The Return of the King
In Peter Jackson's The Lord of the Rings: The Return of the King, Uruks of Mordor are frequently seen: Murgash is one of them, at the Siege of Gondor, and many others are later seen marching through the Orc camp encountered by Sam and Frodo. Most notably, Shagrat, in the Tower of Cirith Ungol, is portrayed as one of the largest Uruk-hai on-screen.

Appearances

 * The Lord of the Rings: The Fellowship of the Ring
 * The Lord of the Rings: The Return of the King
 * The Lord of the Rings: The Return of the King (film)