Jim,

I have seen the Blackjack reference in letters addressed to Henry Marshall, but unfortunately, it doesn't pin down exactly where it is. By 1857, he was living at his plantation at Land's End (when Mrs. Marshall learned that the border was only 16 miles away she said her new home was truly the end of the land. Marshall had promised her after their 1832 marriage not to take her out of the US, which at time ended at the border with Texas.)

I can only guess that it was very near his DeSoto Parish plantation or near Gloster, where he is buried.

I ran across the same dilemmas when trying to locate Red Land in Bossier Parish, Louisiana. Fortunately, some older citizens of Plain Dealing showed me where it was a few years back, which was about 7 miles from them.
Good Luck