Is it possible to black out and lose consciousness during sex?

Not normally: Any activity in which one's nervous system undergoes intense stimulation can conceivably cause a transient period during which you are focused intently on that activity. If, however, you truly lost consciousness that needs a medical evaluation to be sure this is not a cardiac arrhythmia, a neurological abnormality or one of a dozen other conditions. See your pcp asap and best wishes.

Yes: Sexual intercourse requires good working heart, so people known to have heart problems such as coronary artery disease or heart weakness (failure) may not be able to endure the rigors of it and pass out.