This huge, streamlined, slender whale has a broad, flat, U-shaped head and a single prominent median ridge. A small dorsal fin is located about three-quarters of the way along the back. The flippers are pointed and slender, with pale undersides, and the flukes are large and notched. Paired nostrils are located on top of the head with prominent fleshy mounds.
The front part of the mouth is thick with baleen plates; around 300 plates (each around one metre (3.2 ft) long) hang from the upper jaw, running 0.5 m (1.6 ft) back into the mouth. Between 60 and 90 grooves (called ventral pleats) run along the throat parallel to the body. These plates assist with evacuating water from the mouth after lunge feeding.
True to their name, blue whales are an overall bluish gray, although they can appear mottled and blotched. In polar waters, a browny yellow diatomaceous bloom can cover the skin.
Blue Whales most commonly live alone or with one other individual. It is not known whether those that travel in pairs stay together over long periods or form more loose relationships. In locations where there is a high concentration of food, as many as 50 Blue Whales have been seen scattered over a small area. However, they do not form the large close-knit groups seen in other baleen species.