Alan and Len,

Yes, you are both thinking of Article XIII of the 1851 Indiana Constitution, which prevented free African Americans from entering the state and fined both those in violation and any white citizen who gave them employment. Interestingly enough, it coincided with a change in Kentucky law requiring newly emancipated African Americans to leave the state. There was some attempt to enforce this during the war, but later in the war more Hoosiers (black and white) began to support emancipation and openly violated the law.

Still, I think it might be pushing it a bit too far to say all, or even most of Indiana's black recruits were from Kentucky or further South. There were 11,428 black residents in Indiana in 1860, who came to the state before the new constitution, or were born there. It is also worthy to note that the official black enlistment from Indiana might be a little low, because a number of black soldiers from Indiana joined the 54th Massachusetts before Indiana began recruiting African Americans. The site linked below also mentions black Kentuckians enlisting in Indiana, but it does not give any numbers. But I imagine a quick look over the roster of the 28th U.S.C.T. might give some answers.
Best Regards,
Stephen