I would not use the clone tool for this fix; Photoshop's Spot Healing Brush or Healing Brush are far better suited for such a minor repair in an isolated area of the image. These will blend the tones and colours of the surrounding pixels. The Clone Tool transfers pixels and when the colours are not a match, you get the blotched effect you are seeing here. Often a very soft brush helps when cloning as the edges are not sharp and blend in better.
Look at the histogram, especially the black point and white point when adjusting the exposure.