Black shape floating in sky, changing length and direction - Black Flame

I was closing my curtains for the night, the sun hadn't quite set, the sky was blue and orange - I looked above the apartments, about a mile away was this ghost like shape; a solid object at the front, dark black, with a trail behind it like smoke or a black flame. When it moved forwards this trail was longer when it moved backwards there was this glow around it for only a split second. I called my friend from her room and we watched it for 5 minutes before she went to bed not decided on what it was. It danced or swam through the sky for 10 minutes more then vanished right infront of me.