Was shooting pistol today and another guy had some blacked out red dot.  Never seen that before but apparently it's something that you put on your red dot that makes it so that you only see the red dot and not that target. Seemed interesting.

"blacked" out?  Maybe obscured everything but the dot.  C'mon man.

The old armson Occluded Eye Gunsite (OEG) worked on the same principle
Keep 2 eyes open dominate eye “looks” at the black of a black tube with a red dot , non dominant eye looks at the target, the dot is superimposed onto the target
Only works with both eyes open
Back when I was working, I’d often go from bright day light into a darkened house and kept the my the dot on my Aimpoint set about the mid range in brightness and the front lens cover closed. Out doors I’d use it like a OEG, indoors I’d pop the front cover and use it like normal.

Bindon aiming concept

You know what I'm talking about. Looking through the red dot everything was blacked out except for the red dot. I can't remember what he called it. Matriculatir (I know that's not the actual name) or something like that. Just never seen something like that before.

Occluded eye aiming. Look it up, it has merit. What if you're somewhere where debris has blocked your front dot window such as mud or snow on your EDC? Or you are in a two way rifle fight and the other has the sun or some other light source advantage over you so you cover your dot on your rifle with it's flip cap?

Occluding your dot is primarily to ether verify, or train "target focus."  You see the dot with your dominate eye, and the target with your non-dominate eye, which is exactly what you want. If all you see is the dot, you are "Dot focused" which is exactly what you don't want. Some people will shoot USPSA matches with the dot occluded, some people only dry fire with the dot occluded, and some people do both. Either way, it works.