To fit under the seat in front of you, your item must not exceed:
For our Airbus A321:
17″ (43.2 centimeters) length x 13″ (33 centimeters) width x 11″ (27.9 centimeters) height
For our Airbus A320:
18″ (45.72 cm) length x 15″ (38.1 cm) width x 8″ (20.32 cm) height
In order to fit in the overhead bin, your carry-on bag must not exceed:
For all our aircraft types: 24″ L (60.96 cm) x 16″ W (40.64 cm) x 10″ H (25.4 cm)
So it should comfortably fit in the overhead locker, but not under the seat. I would say you'd be fine but I've never flown Jet Blue so I don't know how strict they are - if it's a full flight and the overhead lockers are full when you board then it could be an issue.