Kosciusko, Mississippi is the birthplace of James Meredith and Oprah Winfrey. The population was 7,402 at the 2010 census. It's not far off U.S. Highway 51, about halfway between Jackson and Greenwood, and a little to the east of Yazoo City. It's right in the middle of the Mississippi Delta, "the land where the blues began." I worked as a federal election observer in that area one November in the 1980s. Interesting area, a strange mix of wealth and poverty.

There's an interesting post here (click), with lyrics that are just a bit different.

Blake does seem to write from his life experiences. Ginseng Sullivan was a real guy. The many places he mentions in songs are from where he grew up. So, it would not surprise me if he had an Uncle Sam in Mississippi who was a coke head, lead a brass band, and died. I looked at the words in my Blake Anthology song book to see if perhaps it was veiled statement about the USA, but it doesn't appear to be.
For those who would like to hear the song and read the words:

UNCLE SAM
(Norman Blake)

Mississippi morning, sunshine bound
Somewhere 'round Kosciusko town
Cocaine horse rode on the wind
Slept* last night at the table again

CHORUS
Who's that yonder waving his hand?
Calling out the orders to the big brass band
Playing all the music fine and grand
Let me tell you, good people, that's my Uncle Sam

He had no friends, he had no wife
Snow white cocaine ruled his life
A silver cornet it was his friend
Every** Saturday in the town square band

Mississippi morning, sunshine bound
Somewhere 'round Kosciusko town
The big brass band won't play again
But listen close to the evenin' wind.

https://www.youtube.com/watch?v=bqk66kJHbh8
*I hear "He slept"
**I hear "each" Saturday.