Neither the black box (flight data recorder) nor the cockpit flight recorder will be of much value in determining the cause. They can only eliminate aircraft malfunction.
They will have the pilots voices on them. With any luck they will confirm that they were struck and that they didn’t commit suicide (as has been suggested by several people here) or vice versa.
So then there should be no fight over the black box between Malaysia and the People’s Republic of Donetsk/Russia.