What is the cause of temporary blindness?

Sit down and have a rest and you can recover. Transient blindness is unlikely to be caused by hypoglycemia.

If there is no other obvious discomfort, I suggest you go to the neurology department of a local regular big hospital for an examination, and don't take it lightly. Specific examination items suggest that EEG and head CT exclude the possibility of epilepsy and intracranial parasites. The specific treatment should wait until the cause is found out before targeted treatment.

For reference only.