I'm not sure why ITA shows an A320. It's very possible that JetBlue's less-than-stellar system is reporting an A320 (I've seen the aircraft type on JetBlue's website as "A320" when booking Mint) If it lets you book in Mint, you're going to have a Mint configured A321.
Mint is JetBlue's lie-flat transcon product. JetBlue doesn't describe it as First or Business, just Mint. If you're looking for seat maps, you'll see it as either First or Business depending on what the site feels like rating it.
It's a premium cabin with lie-flat seats. They serve a (very good) meal, comes with two checked bags, and other amenities. The premium cabin is only one cabin, but has two types of seats, single suite seats, and paired seats. The suite seats exist because they needed to stagger the seating for space. (the footwell for the suite seat is under the center armrest column for the paired seats and vice versa)