Personally for the backing I would go with a dark to Navy blue...
Then I would do a few borders...
Instead of pieced borders I would do a thin, thin blue (1"). Then a thin white (2-2.5"), then another larger blue (3-4") then another thin white(2-2.5") then the widest Blue(4-6")...
Just my 2 stiches...
I love it...
K