A Swedish black metal band sprayed pigs blood on concertgoers during a Satanic ritual ... triggering a vomit-fest in the audience ... and we got the DISGUSTING footage.

Watain --  whose members are devout devil worshipers -- played their only U.S. summer gig Sunday night in Brooklyn ... and they were true to form.

You see the lead singer present the audience with a goat skull full of animal blood ... then drench the crowd.  According to interviews with the band, they use REAL pig's blood.

You hear audience members shriek ... and we're told some barfed and cried.

Shockingly ... cops say no one complained.  And no one has heard a peep from the health dept.