Honestly, I'm not sure it will tone it down much to add another fabric to the mix. I say "go for wild & crazy" & keep it as is and maybe if you have enough of the blue print left, bind the whole thing in blue!
If you do want to add another border, I'd maybe add a stop border in solid black & bind in the same color. That would help off-set the black print. Or if you truly do want to tone it way down, change the border out for a solid white & bind in any one of those 3 fabrics (red, white or blue) -- or all three.