Well it's supposed to emit signals for up to 30 days but i wonder if it survived the lightning strike, or if one of its many functions did not die out. I mean sure the black box is said to be indestructible, immortal almost. But if the strike was powerful enough to blow up a plane, surely no one can say for 100% sure that the black box came out unscathed.
Been reading up on the black box a bit, apparently electromagnetic resistance is not even mentioned in its wikipedia page. The black box is tested intensively to survive insane air/water pressure, heat levels, crash impacts and basically anything destructive. Except for something such as an insanely powerful lightning strike, which seems to be the case here.