I sent this question to JB and here is their response:

If you notice a lower fare on your same flight within 14 days of your original booking date, let us
know by promptly calling 1-800-JETBLUE (538-2583) and we'll be happy to offer you JetBlue credit for
the full difference in fare. If you notice a lower fare 15 days or more after your original booking
date, we'll still honor the lower fare for a fee of $60 - $70 per person if the Blue or Blue Plus
fares were selected at the time of purchase. If you purchased a Blue Flex fare, we'll waive the
Honor Lower Fare fee and you'll receive the full difference in fare currently offered, even if you
notify us of the lower fare 15 days or more after your original booking.