Blue Ensign of South Africa from 1910 until 1928.  (Unlike the Red Ensign, the Blue Ensign was not altered by placing the shield on a white roundel.  The version of the shield shown in this image is not contemporary : it is the 1932 rendition of the shield.)
This file was derived from: Red Ensign of South Africa 1910-1912.svg:
South_Africa_Flag_1910-1912.svg: Fornax
derivative work: Suzuki Auto