I did quite a bit of research on the issue myself. As soon as I arrived at the check in counter, I can confirm that was asked about a return ticket.
Initially I was considering a bus ticket, or worse case booking last minute at Expedia, and taking advantage of the 24 hour cancellation. For peace of mind, to avoid a hassle, and tying up funds for months...I ordered a 48 hour return ticket for $14.
The website provides an actual return ticket, which can be verified on JetBlue's website, with a confirmation and itinerary number. You have to purchase it the day before to provide time to process the flight and allow a time window. Believe you can also book in advance. In my case, they confirmed that I had a return flight and handed me my boarding pass https://flyonward.com/en/