Besides, the "South" that is becoming part of the "Democratic wave" is the Atlantic seaboard, including South Carolina, not all of the South. Clearly Landrieu was the only vulnerable incumbent Democrat and merely did ok while Obama did far worse than expected in every Southern state without a sizable black population. Look at Missouri or even Arkansas.
I'm also not sure how well Landrireu would have done if her last name was something like Landon. She probably still would have won, but there's a rule of Cajun voting that all else being equal you back the Cajun
Seems to be an ever-weakening rule, though...
I suspect that the 2007 election (which is what I think you're referring to) had more to do with guilt over electing a complete incompetent like Blanco because she was Cajun more than any voting shifts.