My cat is only recently partially blind in her left eye

Question
I am wondering what could be the cause of this partial blindness, its only a recent thing, and it appears that there is a vein going from the edge of the eye to the blind area, she is not an old cat, only about 4 yrs old, used to live outside with two other cats. She is a very scared cat and is even more so since the partial blindness, it seems to be more of a bother during the day time..

Answer
Sincere apologies for the delayed response due to my own illness.  Corneal lesions which can impair vision are most often caused by physical trauma (tree branch, rough particle, another animal)with other causes being viruses and abnormally low tear production.  Any corneal injury can lead to blood vessel migration as you have noticed.  Healing can take many weeks so there is the possibility of some of the vision returning.