In 1701 France was threatened with a new outbreak of fighting against the naval powers (England and the Netherlands). This gave rise to fresh fears for the safety of France's coastal towns and harbours. Fort Blanc was built as a result of renewed fears of an Allied naval bombardment of Dunkirk. It addressed a perceived weakness in the defences: the area to the east of the jetties, where a fleet of bomb vessels could theoretically come close enough to bombard the town.
Fort Blanc, which was named after the naval intendant le Blanc, was the only one of the sea forts that was not accessed by a jetty. It was dry for several hours at low tide, so it would have been possible to walk to it, or to reach it by boat at high tide.
The fort was built of stone, with a barrack building on top and vaulted rooms underneath. The entrance was via a stairway that led from the gateway up to the top of the fort, where the guns were located. The main battery of the fort was a wide curve that faced north, out to sea. To the rear there was the gate, covered by loopholes for musketry defence in the outer wall of the vaulted lower storey. The guns at the ends of the main battery could also be turned to face southwards, if an enemy force came between the fort and the shore.