I would agree with your first guess, it sounds like an Indigo snake to me, but it could have been a Black Rat Snake too. The reason it was so fat may indicate that it had just taken a big meal and its body was still distended. The stretching of the skin would also account for the purple appearance (the skin between the black scales shows when the snake is still digesting a big food item, this gives a purple appearance). Neither of these species is poisonous.
