It was like a giant black whole in the sky moving slowly from left to right

The best description i can say is the object looked like a huge black hole, moving from left to right, moving very,very slowly at one point stationary.Im looking at it from street level looking west at the skyline as the sun went down when out from behind the frame of the Willimsburg Bridge in brooklyn it appeared. At first i thought it was a shadow or something but when i saw the planes,and traffic hellicopters flying near it,i knew it was not a shadow. At one point it looked like it changed shape from a circle to a shape of a top hat. I tried to figure out the size of it as i saw a few jet planes flying in its direction you could fit at least 3 of those jets together just for the width,and it would not cover the whole thing.After about 8 minutes of whatching this, the black mass started to move away,and this time at a steady flow forward heading west into the sky line.