Blue Darts are given to jetblue pilots on a random basis. The jetblue pilot can then give the blue dart to a friend who is applying for the job, and that person will be guaranteed an interview (they still have to pass the interview all on their own).

Internal recommendations are basically letters from existing pilots who vouch for the applicant. You aren't guaranteed an interview just because you have an internal rec (but it helps).

Yes, you can have a blue dart and still write letters of recommendation for other people.