On the other hand it could mean that the rebels have nothing to hide and the evidence is already in the hands of the Russians
Who will be able to prove someone manipulated it if the report says something different (although I don't think its easy to manipulate a black box)
Those boxes are sealed. They were reported by the freedom fighters to be in good condition, which probably means that they are not broken, which in turn means that they have no idea what's stored inside. Besides, it's not like you can just open the box and read the data off a harddisk... Expertise at interpreting that data is needed.
As I got it, the information on the memory cards in these boxes is encrypted, so even Boeing itself couldn't fiddle with it.