Forgive my ignorance but, shouldnt the black boxes be positioned in an area of the airplane that somehow once there is an accident could they sort of be released by design onto safe ground and furthermore enclosed in well engineerd floating covers. It seems to me that this is technologically feasible.
Hernan herman, you question is a good one and you will find that there are some Flight Data Recorders which are designed to use the "kinetic" energy of a crash to be propelled away from the crash site, (althogh having a FDR land on you head would finish you off even if the crash did'nt!), newer FDR's are programmed with homing signal transmitters that will send a signal for up to 30 days, however thats if the force of the crash has not damaged the system.
If you read what French avaition officials have been saying they believe the aircraft may be in up to 12,000 feet of water & no FDR has ever been recovered at such a depth, whilst they are built to "take a licking & keep on ticking" the pressure at such depths may well have shattered the casing & rendered the information unreadable.
Google Flight data recorder for a better understanding of how it works.
Hope this helps.
Bruppy