I have now received an email from Jet Blue with a $25 refund, which stated in part..

"In reviewing your reservation, the ticket listed in our system shows a Blue Fare purchased with True Blue points. However, we can see from your scanned reservation that it clearly states you purchased a Blue Plus fare. It is unclear why this was incorrectly printed with your itinerary, but we can understand why you would be confused when you were asked to pay for your checked bag. Please accept our apology for the difficulties this caused." Confused?? Confused, no. Ticked, yes. So, got my money back and I doubt I'll be flying them again soon.

Regardless, anyone flying on Jet Blue, make sure you take the receipt showing you paid for checking a bag, so you won't be "confused" like I was...