I fly to ORD a few times a year and flights are almost always full both ways. JetBlue changes the frequency and aircraft type between JFK-ORD and BOS-ORD depending on the time of year. Usually ranges from 2-5 flights daily.
BOS has always been considered a mini-hub for Delta so I'm not surprised to see them expanding there. I would guess this is targeting BOS based travelers as most Chicago and DC based travelers are loyal to AA, United or WN. DC also makes total sense given JetBlue travelers are annoyed with the closing of IAD. The question is are they annoyed enough to switch loyalty to Delta?