It amazes me how people from Northern NJ, Bronx, Queens, Staten Island, and Brooklyn all have that accent, yet no one on Manhattan does 
I think you have it the opposite way, lol. The accents all sound very similar, except the Brooklyn accent is more distinct and thicker than those from Manhattan and the other places you mentioned. We pronounce our "er's" as "a's". Brooklynites speak it with more force. Another reason why I think I'm having this accent is because most of the people living in the area speak with a New York accent, and so do my professors. My math professor (who I talk to to the most considering my love for math...) speaks with a heavy New York accent and I'm basically forced to listen to everything she says so I can understand the concepts and skills in class. So, in a way I have not only learned the concepts that she has showed me but her accent, too.