The Blue Band Squadron of Hork-Bajir Controllers are the elite fighting force guards of the newly-minted Visser One, and are named that way because they wear blue bands on their upper arms. Their leader is called Grath.
The Revelation - A Blue Band Hork-Bajir threatens Rachel at the Yeerk Pool
The Diversion - The Animorphs morph Hork-Bajir to infiltrate the Controller-run Midtown Bio-Services lab. They use Marco's blue-colored T-shirt to pose as Blue Band Hork-Bajir, so that no other Controllers would question what they were doing.