A black box isn't going to be of as much use as access to the crash site. Bits of missile warhead buried in the skin panels are the smoking gun, and should clarify which missile hit it.
The KAL007 CVR and FDR was hidden in a safe of a Soviet official in the Kremlin until the Soviet Union broke up. Until it was handed over and analyzed, the West had no idea of what happened to the aircraft in terms of the breakup.
It turned out the cockpit crew was alive after the missile strike and was trying their best to control the aircraft. They also heard cabin announcements in the CVR which led them to believe that the aircraft was relatively intact after the missile strike. The passengers and crew suffered.
Unfortunately, the CVR and FDR are probably on the way to Moscow right now, probably never to be seen again for another generation.
God Bless General Patton. He had the right idea in 1945 after conquering western Nazi Germany, the Allies should've pushed on to Moscow.