Cloudiness occurs when iron and tannins in the vinegar combine.
Tannins are responsible for the stringent flavor in the vinegar. The reaction is more likely if the vinegar is stored in a clear bottle and is exposed to light. Acidity of the vinegar is not affected by this color reaction. Darkening of vinegar may also just be an effect of age. This low darkening is the same thing as the darkening of fruits with aging.