New York City was home to a little over 8 million people in 2003. If Oskar met them all, we'd have to change the title to Extremely Loud and Incredibly Close and Impossibly Long. But there are a few notable citizens who help Oskar along the way, and they aren't even all named Black.
While space constraints prohibit Shmoop from giving you a run-down of all the Blacks, here's a quick summary on just a few of the minor ones Oskar meets along the way. Aaron can't leave his apartment. Abe rides the Cyclone. Ada is the "467th-richest person in the world" (7.49). Alice seems to be a squatter. Allen needs help with his e-mail. Arnold isn't any help. Fo is a Chinese man in an I ♥ NY shirt. Mark is crying when he opens the door because he always hopes that whoever is knocking is the person he's looking for "even though he knew he shouldn't hope" (13.70). Georgia lives on Staten Island with her husband. Ray is in prison for raping and murdering two children (good thing Oskar's search happens after this man is caught). Ruth lives on top of the Empire State Building since her husband died and catches the eye of Mr. Black.
Gerald's the limo driver who doesn't just take Oskar to his Dad's fake funeral with the empty coffin. He also takes him to dig up the grave and even mans the shovel himself. His limo lends itself to a little bit of existential thinking from Oskar:
"If limousines were extremely long, they wouldn't need drivers. You could just get in the back seat, walk through the limousine, and then get out of the front seat, which would be where you wanted to go." (1.11)
He's the Nelson Muntz to Oskar's Bart Simpson. He makes fun of the way Oskar's Grandma reacts to the school play, calls Oskar weird, and makes him say that his mother's a whore. Oskar does stand up to him at the school play, telling him "You are guilty of having abused those less strong than you" (7.32) in a moment that would make Wednesday Addams proud. Except, unlike Wednesday, Oskar just does it in his head.
This guy should really be a sitcom character with that name. His job is mainly to tell Oskar "You've got mail!" in the AOL voice. On the worst day, he gently pats Oskar's cheek.
Walt's the locksmith, who admits that his job is endangered (like the USPS or a cobbler) but he accepts it and even offers Oskar a little bit a wisdom about having a unique profession:
"We're useful now, but soon we'll be interesting." (3.13)