Yeah I have doubts whether he could win the primary, and I have bigger doubts whether Blackburn will beat Bredesen.
In the past decade or so, the southern white vote has gotten way more Republican. The Weekly Standard on this subject:
http://www.weeklystandard.com/could-the-democrats-and-phil-bredesen-flip-tennessee-2018/article/2011014
Moreover, as Governing has documented, not every red-state Democrat who performed well in the past has been able to replicate their performance in the new political environment. In 2016, Evan Bayh, a former Democratic senator and governor from Indiana, came out of political retirement to run for his old Senate seat. He earned more votes than Hillary Clinton did in Indiana, but still lost by ten points. Bob Kerrey, a former Nebraska Democratic Governor and Senator who left the upper chamber in 2001, tried to run again in 2012. He got a larger share of the vote than Obama, but also lost by double digits. Ted Strickland of Ohio is a slightly different case because he lost his 2010 gubernatorial re-election bid before failing to unseat Republican Rob Portman in 2016—but it’s still not a great precedent for Democrats.
Bredesen has a lot of political skeletons in his closet as governor of TN and mayor of Nashville. Only the hardcore demorats will support him. He ain’t from around here... grew up in NY... he will fit right in with Schumer.