Those might have been crushed by a skyscraper falling on top of them. A fire and a crash won't come close to destroying a black box.

Well, 9/11 is a different story. Like they have found the passports of the hijackers but the black boxes were destroyed. And some hijackers were alive after the event http://news.bbc.co.uk/2/hi/1559151.stm and also not forget that there are too many problems with the official story which makes it suspicious.

You might find this site useful: http://www.911myths.com/html/hijackers.html