What causes grey circle in the line of vision while blinking?
Possible vitreous degeneration of eye
I can certainly understand your concern.
I have worked through your query in detail.
As per your description, this football sized grey circle with black borders possibly represent vitreous degeneration.
At this point, it's pretty important to get a Fundoscopy examination done to assess the posterior structures of eye and to take further action.
Indirect ophthalmoscopy will yield great diagnostic results
Post your further queries if any.