I'm sure the polls have been looking bad along with a lot of bad publicity over the past year or so factored into the decision. My guess is that nothing big was promised but a meeting with the party leaders or info from them convinced her she wasn't going to be backed by the party. Remember a couple of weeks ago it was announced that John Breaux may seek the governorship but he'd not make any decision/announcement until after a face to face with Blanco. I don't know that the meeting occurred but I'm sure such a meeting was weighing on Blanco's mind and thus the decision not to run.
Breaux may not run or even want to run and the threat to run may have been a ploy to get Blanco to step out so there wouldn't be an incumbent from their party running. Speculation on my part only.