This was quite possibly a transient ischemic attack
or a ministroke. He needs to return to the physician who is treating him for his hypertension
for drug adjustment. If the doctor is not able to see you TODAY, then report to the Emergency Room Immediately. There may also be some additional drugs to be added to prevent future events. If managed correctly, a future stroke which can permanently take his vision can be avoided.
Please read this article on TIAs and transient blindness
, or run a google search with "TIA blindness"