JetBlue usually sells inflexible tickets with fewer connections, so passengers are much more likely to show up for their flight. In 2013, they only had to deny 18/21,000,000 passengers where the average rate is at 9/10,000.
No. JetBlue doesn't overbook. Starting in Q3-4 of 2016 JetBlue starting having fleet issues with the A321s and many times had to swap in a A320 (with ~50 fewer seats) due to mechanical issues with the newer A321s, which resulted in a massive spike in involuntary denied boardings. You can definitely argue that if you get bumped from your flight the cause doesn't matter much, but JetBlue apparently does NOT overbook at all... Which I respect.
https://www.youtube.com/watch?v=EqWksuyry5w does a great 5 min overview talking about this exact thing.