I do not know if this is the same person, but there is a Private Richard Balshaw, Company I, 1st Louisiana (U.S.) Infantry on a list of "Officers and men who volunteered for storming party at Port Hudson, La., under General Orders, No. 49, Headquarters Department of the Gulf, June 15, 1863" that I found in Series 1, Volume 26, Part 1 of the Official Records of the War of the Rebellion. I do know that those assaults were particularly devastating and that lots of men died in the attempt. Given the dates that he disappears from the Confederate records, it is plausible that this could be the same soldier who was captured and joined the Union army. Unfortunately, I could not find his name in the National Archives Civil War Soldiers and Sailors Database on the Union side, only the Confederate side. If I had to venture a guess, it would be that he joined the Confederate army, was captured and offered the alternative of joining the Union army rather than going to prison. If that is the case, I would say that he died in that assault since there is an absence of any further records on him in the Union army.
I hope this may clear things up a little.