Recall the bubble from the beginning of the poem when the speaker wants to pop the bubbles blowing from the fish mouths in the aquarium.
This bubble also reminds us of the previous line with the balloons.
This is a fantasy-like image, because—obviously—Colonel Shaw is now dead, and even alive it would be pretty tough to ride a bubble. We'll have to read on to see if the meaning of this image will unveil itself.
he waits for the blessed break.
Hmm, not quite the decoding we were hoping for, but let's take a stab. Colonel Shaw's riding on an imaginary (or figurative) bubble, waiting for it to break.
The bubble could stand for the cause or the struggle (a united country with united countrymen, regardless of race) that he and his regiment fought for.
The break could also mean the relief (as in, "give me a break!"). Because the Civil Rights struggle is still going on during the time of this poem, maybe the speaker imagines Shaw waiting for it all to be finally resolved.