I think Sueanne Weaver (season 1) was a precursor to Jo. She would tell Blair off sometimes and in one episode, she punched Blair (on accident).

The Blair character really needed a foil or antagonist. She was too rich, too beautiful, too intelligent, and got what she wanted too easily. The other Season 1 girls were too much like her to give Blair any significant challenges or provide a meaningful contrast in character. Sue Ann the farmgirl wasn't really different enough to work as a good foil to Blair. I think you're right, though, in that the writers tried to use her that way a little.