Rorquals are differentiated from other baleen whales by having dorsal fins and long grooves along the throat. The grooves act like pleats, allowing the throat area to balloon out with water when the whale is feeding. Blue whales have from 55 to 88 of these grooves, some extending from the chin to the navel. Other species in the rorqual group include the fin whale, sei whale, Brydes whale, minke whale, and humpback whale. Fully grown females are usually larger than males.
Blue whales are blue-gray in color with lighter mottling on the sides and back.
The darkness of the color and the amount of mottling can vary between animals. The undersides of the fins are white. Growth of diatoms on the belly area of blue whales may give a yellowish color that has caused them to be called sulphur-bottom whales. The head of a blue whale, when viewed from above, is wide and flat, somewhat like a gothic arch. When breathing, a blue whale can spout water up to 25 to 30 feet in the air.