It is not the Confederate Battle Flag!. It is the Confederate Navy Flag. The Confederate Battle Flag is square and the diagonal blue strips are dark navy blue.
Just a point of correction.
Wasn't just the naval jack. Here is some history for you about the origin of the flag and some units that flew it: Battle Flag of the Army of Tennessee. The Army of Tennessee was the second largest Confederate army. Continue scrolling down and see also the Battle Flag of the Department of Alabama, Mississippi, and East Louisiana.
It is the 2nd Confederate Naval Jack (1st official) adopted in 1863.
The Confederate Navy Ensigns were the 1st National (pre 83) and the 2nd National after it was adopted in 1883 at which time the Official Navy Jack (the Rectangular CBF) was adopted as the jack.