There are many reasons for your mom to be seeing this black spot in her vision. One of the most common reason is called Posterior vitreous detachment
or PVD. In everyday terms it's called floaters
. Our eyeball has this jelly substance which keeps it inflated, kinda like a balloon. The back surface of the jelly is attached to the retina
and the front surface is attached to the natural lens
in the eye. Now, as we age, the back surface of the jelly will peal off leading to this floater or black spot looking thing that patients usually see. They are seen when starring at the clear blue sky or a white/pale sheet. This will resolve on it's own HOWEVER it can stay for a long time causing visual disturbances. There is no cure for it!
The other reason could be that there is a bleed inside the jelly and she is seeing the blood. If this is the case, then she need to get her blood work evaluated soon.
The other option is that she might have an inflammation in the back in which she also needs to get her blood work done to get that taken care of.
Hope this helps,
Dr. Jay Patel