After completing all objectives or inflicting total damage to this racket, talk to Bear to know Sonny Blue’s location.
Sonny will be in the slaughterhouse where you rescued Ralph earlier. There are a lot more enemies now but you can use the same tactic and entry point as before. Sonny will be in the same room where Ralph was tied earlier and when you’re near enough to overhear their conversation, he’ll move to his office. There, you can lob an explosive or Molotov then unload your weapon to finish off the job.
Securing this racket will increase Burke’s income, give you a marker, and upgrade the associate / favor level. “Cops nearby” will mark nearby cops on your map.