It is a black rat snake. It is not poisonous. The black racer looks similar (white under the head, but black under the body) and is not poisonous either.
What Snake Is Black With A White Belly That Has Black Spots?
What I found is a northern black racer. From the details you provided I could look at the pictures of snakes and find the right one. It is poisonous. Very. Hope this helps.