Terminal C in BOS is already very well serviced by several Priority Pass lounges. Yes, "The Lounge" isn't great, but its not the worst either as PP lounges go. Air France is OK. Chase Sapphire, though, is in a league of its own. Great food, great bar, great amenities. Given that one can get unlimited access to Chase Sapphire (self + 2 guests) for net $250/year after the travel credit, the JetBlue premium card would have to be pretty inexpensive for JetBlue to beat what is offered there. Or... the JetBlue lounge would need to be better than Chase Sapphire, which is a pretty high bar.