Dominican immigration will certainly not ask for it even though its technically required, but the airline might ask you if you are also a DR citizen or resident if you do not have a return flight with them. You should show them the confirmation from the other airline for your return flight and you should be fine.
Buried deep in JetBlue's website is their policy on the issue:
US Citizens traveling to the Dominican Republic
Proof of return or onward travel is required at check-in. Failure to provide such documentation will result in denied boarding.
Passengers exempt from providing proof of return or onward travel are:
-Customers traveling with a B1/B2 visa who may travel one way
-U.S. citizens born in the Dominican Republic
-Foreign minors traveling with parents who are Dominican nationals.
-Legal residents of the Dominican Republic traveling with a valid re-entry permit.