GeorgeC1 - OP says they only have carryon so no need tp worry about having luggage checked through to final destination. They can simply deplane from their Jet Blue flight which will arrive in Terminal (Concourse) C and follow the overhead signs to the departure gate for their Cape Air flight which will be in Terminal (Concourse) D. They will not have to leave the secure area. It should be less than a 10 minute walk between gates.