The "Stop Snitchin'" campaign helps them, but most African-American Muslims have been through the justice system and thus there is a lot more data on them than there is on illegal Arabs with fake identity papers. The chances of there being an entirely African-American terror cell with no ex-convicts in it is about a billion to one.
If you get just one name, one ID, then you will be able to see who the terrorist was in prison with, which imam he is linked to, etc. It will also be easier to find out where he works, if he works, who his associates in licit or illicit businesses might be. Very few newly-converted Muslim ex-cons have enough personal cashflow to fund terrorism operations.
And it will soon become obvious to any other criminal in the African-American community that credible information regarding terrorist activity is the equivalent of a get-out-of-jail-free card.
"Michael, this is your third offense - you're looking at a 10 year bid for possession with intent to distribute. Is there anything, any information at all, that you think might help with your sentencing?"
"If you make sure I don't have to deal with any Muslims in the joint, I may know something . . ."
"If you know enough you might not have to worry about prison at all."
Those are very good points. AQ has already tried working with alienated blacks (such as in Portland and Washington State), but one of the things that has done in their efforts has been the fact that almost all of the people involved had prison records and were or had been under surveillance for other things (not related to terrorism).
If they're looking for "lily whites" - sorry, there was just no other way to say it! - they won't find them in the black male population. And I also really doubt that they'd find many folks in that group willing to carry out suicide operations, either.