Interesting things about this story.
Who, outside of a specially trained person, would be able to idetify a "black box" in all that rubble? I wouldn't recognize one if it was sitting on my desk. I mean, was it labeled "Black Box"?
So, an FBI agent takes something from you that you believe is VERY important and you don't even bother to get his name?
Other than that, this is a hugh, series story.
The CVR and FDR in aircraft are fairly easy to recognize. They're painted bright orange and are labeled. However, they are not designed to survive a 2,200 degrees fire for more than a few minutes. While it is certain they survived the impact, they were incinerated in the ensuing fire which was hot enough to melt the large steel rod in the center of the WTC.
It is identifiable as an object rather than a piece of trashed rubble. It is not destroyed, but clearly still in one piece. It is something a person would pick up and say, hmm, what could this be?
No, they're not labeled "black box". And besides, they are optical orange.