Blacc Sam is singing Jay-Z's praises for honoring Nipsey Hussle on stage ... telling us it was a "monumental" gesture from someone they looked up to since they were kids.

We got the late rapper's brother at LAX on Monday ... and he told us his phone was blowing up after Jay's concert at SoFi Stadium over the weekend in Inglewood, where HOV paid homage to Nip.

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Sam says Jay's done so much for him and Nipsey over the years that people just don't know ... from a music standpoint and a business side.

He says Jay went from someone they idolized and looked up to, to someone actually in their circle ... and really championed Nipsey's career.

According to Sam, Jay's been a huge help supporting Hussle, their family, and even the Marathon Burger chain he co-owns -- which just opened a new location at LAX in Terminal 1.

The marathon continues ... and Jay's doing his part.