Dark shadow in vision may refer to scotoma(blind spot
) which normally present in all. It cannot be noticed. Pathological dark shadow which you are talking about could be scotomata and the problem of retina
or optic nerve
. If you are a Hypertensive please control your Blood pressure.
Scotomata could also be due to nutritional imbalances, vascular blockages. You can consult a retinal specialist as referred by your ophthalmologist for the specific treatment.
Have a Good Health.