The Carroll County Blues is indeed about Carroll County MS; we know that because that's where Willie Narmour came from. In fact, Harry Bolick (and I'm sorry I can't do a blue ckicky thing to direct you to his website) will sell you a beautiful T-shirt with a picture of Mr. Narmour against a background of the state of Mississippi, with Carroll County outlined.
In the US, there are LOT of counties in the East named for Revolutionary War heroes; ther's at leat four Montgomery Counties that I know of, all for the guy who got himself killed when the US tried to invate Quebec. Carroll I assume to be for Charles Carroll of Carrolton, MD, last survivor of those who signed the Declaration of Independence.