Welcome to HCM.
You have experienced a TIA
( transient ischaemic attack) which is a due to sudden transient loss of blood supply to your retina
causing a blackout
Since you haven't mentioned your age, I am suggesting in general how to proceed further.
Get your Blood pressure, blood sugar and lipid profile
done along with an ECG. If it shows significant changes ,a Carotid Doppler ,in specific will help your ophthalmologist to conclude the diagnosis and then proceed with the treatment.
So get an ophthalmologist Retina opinion and a through investigation to avoid future recurrence. Take care :).