yeah, this has happened to me.. three times in my life.. but it's not just a few seconds of black-out.. but more like 30seconds to a few minutes...
the first time it happened was in gr 7, i was really sick and all i did was sleep all day, but my sister wanted me to get some fresh air, so we were at mcdonald's, and then everything started getting really bright? like white wasn't really white anymore, and then i blacked out.. and i thought i went blind because my eyes were open, and i could hear voices and stuff, but i couldn't see. my sis wanted to take me to ER too.. so embarassing because everyone in mcdonald's was crowding around me

but a few moments later, my vision came back. yeahh... it scared me a lot.
the second time was when i went to taiwan and we were in a temple. i wasn't feeling well that day either, plus i think i didn't sleep the night before. and i blacked out a bit too... and i think it was maybe in that same year..
and i haven't had it in the longest time. but recently on boxing day, i didn't sleep the night before, so while we were lining up, i had the feeling that i was going to black out, so i went to sit and rest for a bit.. and it helped :/
i think it's just low blood pressure.. also getting enough rest for your body.
my sister said because of low sugars? so that's why on boxing day, they gave me candy to eat, haha.