What you are experiencing is due to TIA( transient ischaemic attacks) in the retinal blood vessels.
It could be due to blocking of the bloodflow ( by a clot) through the retina causing a temporary vision loss
which restores when the clot moves away.
So it is a concern if blackening of vision is recurrent.
However confirm this by a direct retinal examination and an ECG and MRI
brain to rule out blood clots
. Get a cardiologist opinion too.