Wiktionary
n. (context historical American Civil War English) a Union soldier
Usage examples of "blue belly".
The huge, wrinkled-blue belly of the Indian Ocean dominated the planet, with the spiral of a big swirling anticyclone painted across it.
A second later, after two or three bullets cracked past his head, he was behind that rock, with no company but a little lizard with a blue belly that scurried off into the leaves when he thudded down.
The Colonel spoke no English, but he was fluent, dreadfully fluent, in French, a language in which Jack could at least listen and in which, mindful of Stephen's warning that their guest must be handled with the greatest care, he had listened for hours and hours, doing his best to follow and in the rare pauses contributing remarks that might perhaps be appropriate and which he knew to be French, such as holy name of a dog, look me that, and blue belly - hours in which Stephen had deserted him to swim in the newly-recovered Catalanity of the transports.