The basic design of the Blackburn Dart was developed into a two-seater to meet a Greek Navy requirement for a coastal defence seaplane.

The Velos differed from the standard Dart T.2 in having a two-seat cockpit with a rear-mounted .303 in (7.7 mm) Lewis Gun, an increased weapons load with four 230 lb (104 kg) bombs mounted under the wings and provisions to fly as either a seaplane with floats or with a conventional undercarriage.

Blackburn produced two additional T.3 models as the T.3A Velos, initially for trials of the company's new metal floats and later, one example embarked on a demonstration and sales tour of South America in 1927.

The Blackburn T.3 Velos not only fulfilled an operational role as a coastal defence/torpedo bomber in the Naval Air Component Squadrons in Greece, but also helped establish an indigenous aviation industry.

The aircraft began operations in 1926 with the Greek Navy deployed at Tatoi Aerodrome and Phaleron Bay, Athens.