Definition of "blackouts"

[…] her blackouts. When the Officer inspected everything went well. The only thing was that two of our WAAF were not wearing any knickers at all!

1995, Beryl E. Escott, Our Wartime Days: The WAAF in World War 2, page 35

We had what were called the ‘blackouts’ and ‘twilights’. The blackouts were a dark sort of long-legged knickers, dark wintry ones, and the twilights were a lighter blue in a sort of Celanese material.

2011 December 13, Peter Liddle, Captured Memories, 1930–1945: Across the Threshold of War: The Thirties and the War‎, Casemate Publishers