The Civil War Soldiers and Sailors index system does list a John Keller 16yo from Ouachita Parish Louisiana serving on the ship, Fearnot, enlistment date May 16, 1864, as a First Class Boy. I'm not certain if the following information is the same John Keller:
February 13th, 1867 - Captain J. Amrean, Opelousas, reports that John Keller, freedman, who lived with Actheon Carriere, Constable of town of Washington, asked permission to go and see his wife who was sick. His employer refused, threatening to shoot him if he left the house. About two days afterwards, his wife sending for him again, Keller took his employer's horse, was folled followed by Carriere, who coming up to freedman shot him five times, then tied him on a horse and brought him back to Washington repeating striking him repeatedly on the way with a heavy stick. Keller made no resistance and from his dying declaration it is a clear case of murder.
Looking at the Census information Actheon Carriere lived in Orleans parish prior to 1867. The only other possible hit on Census info was a "John Kidder", not "John Keller".
The Freedman's Bureau did have a document, but it wasn't clear if this was the same John Keller due to the fact that the Freedman's information stated that John Keller had a "mistress" in Kentucky who was a widow. Paris, Kentucky.
It was interesting to note the thread and take a look see.