From 1858 to 1866 an Admiral of the US Navy would fly a flag of a solid colour of either blue, white, or red, to show their rank, blue was the colour used by senior admirals, whilst commodores flew a swallowtail pennant.
It appears that this practice was also adopted by the Confederate States Navy, because Admiral Franklin Buchanan flew a plain blue flag besides his ensign and pennant from the CSS Tennessee, his flagship, during the Battle of Mobile Bay of the 5th of August, 1864.
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