Blackbox would intentionally be "not found" in such case, it would be counterproductive to allow it to be located or analyzed.
MY research forces me to personally believe 93 was shot down by Maj. Rick Gibney of the "flying hooligans", and it was destined for wtc7 [explains why it was wired to blow prior, and had to be pulled no matter what.]
(If the crash site in shanksville is real or staged is another subject, another crash site could have been covered up easily with all the sensationalism in the news that day)