Every pilot that flies high performance aircraft experiences "greying" in their vision under G from time to time. The hazard in front line fighters is that they can normally pull well in excess of 7.5 G's so quickly that the grey out warning never happens.
That's right. The thing with jets like the T-37 is that it doesn't pull high Gs, so guys don't take it seriously and do anything to prepare for it. Lack of experience, rapid onset of G, and doing nothing to mitigate it causes problems with trainees. The instructors take control after the student goes to sleep. After they wake up, he says to them, "If this was a single-seat jet, you'd be a pink puddle at the bottom of a smoking hole."
That's what happened on this one, there's an article about it. He didn't experience the usual seeing stars at the edge of his vision or things going gray, he just suddenly went out. He woke up with his nose over the horizon and hearing the flight lead saying, "Knock it off..."