The new flag had a gold lightning flash passing through a central gold fasces on a black background. This large flag, which was made of cotton material, was used extensively from 1933 up until the disbanding of the movement in 1940.
In the gif image below, a black Fascist flag flies alongside the Union Jack at a B.U.F. rally in Manchester in the 1930s. The flag is at the head of the column, just behind Mosley.
B.U.F. Rally, Manchester 1930s