I was thinking pontificating the idea that "blacking-out" can be a result of restricting blood flow in the case of the leg press, gripping too tight and the compression forces produced by sitting also restricting blood flow, resulting in an increase in blood pressure.
One disadvantage of the leg press (when moving heavy) is the extremely heavy force and low rep's applied on the hip as well as compression force on the spine, when you fatigue you also tend to lose spinal integrity very quickly.
I find with myself over many years of leg work that it is the hip that take a shot big time, as Ive gotten older i find my legs fatigue very quick.
Learn to identify and respect your limitations instead of always pushing them.
Anyone can learn how to push hard, but few bother to practice restraint.
Knowing when to accelerate and when to hit the brake are what will keep you in this game for the long haul.