"Allen Black lived on the Lower East Side and was a doorman for the building on Central Park South, which was where we found him." (199)
-- The word black occurs many times in this book. Oskar, is on a quest to search for a person with the name black that may know something about the lock to the key that he is searching for. He is very looking for this because he found the key with a letter that had the name Black on it. This is why he is on such a great quest, and the word black is very important.
The word black written on the envelope in red: black is the antithesis of white. And once something is stained red, it is almost impossible to get it out. This idea may be very off-base, but it could symbolize Oskar stepping out of his comfort zone. He is trying to find all of the Blacks in New York City, and meeting new strangers isn't too inviting for him (nor is it for anyone really). The red remains somewhat of a mystery, but it is still a clue that will lead him to his answer.
Oskar speaks of a bed/tree when he goes to Mr. Black's house, and this could be a reference to the giant bed carved from a tree in The Odyssey.
One interesting thing with the blacks is Oskar's feeling when confronted by the old man. When he is there Oskar feels a little bit closer to discovering the key that will unlock the significance of doornobs and get him coloser to his goal, but when he is gone Oskar feels lost again.