I agree that it is confusing. One difference between a refundable class fare and a Blue Flex fare is that with a refundable fare, you can actually no-show for the flight you booked and get a credit of the full amount you paid. Here is what B6 says about all non-refundable fares (including Blue Flex fares):
If you do not change/cancel before scheduled departure and you "no show" all money for reservation is forfeited.
And here is what B6 says about refundable fares:
All customers MUST change/cancel their flight prior to departure in order to receive a refund. If a customer "no shows" for a flight, the money on the reservation will be placed into a JetBlue credit for future travel within one year.
There are other advantages to booking Blue Flex vs. say a regular Blue refundable fare including:
- 2 free checked bags
- 5 bonus TB points vs. 3
- Expedited security
As a Mosaic member, almost none of this matters since you can book a regular Blue fare and pretty much have all the benefits of refundable/Blue Flex other than being able to no-show and still get a credit. In short, Blue Flex gives you 90%+ of the benefits of purchasing a Blue refundable fare, plus the other ancillary benefits above, but you need to cancel if you're not going to make the flight, otherwise, you forfeit the money paid. If you are not Mosaic, it really depends on how much you value each of the benefits provided by each fare class and what the price differential is.