Blake Painter -- best known for being a captain on seasons 2 and 3 of "Deadliest Catch" -- has died, and police are testing substances found at the scene of his death.

Law enforcement sources tell TMZ Painter's body was discovered Friday in his Oregon home. We're told a friend of Painter's became concerned after not hearing from him for a few days ... so he called the police. Officers gained entry, and determined Painter had been dead for several days.

We're told cops discovered substances inside the home that they'll test for possible narcotics. They also found prescription pills, but it's unclear if they were a factor.

No foul play is suspected in his death, and an autopsy and toxicology test will be done to determine the official cause.

Painter was the captain of the F/V Maverick on the hit Discovery Channel show, and appeared in season 3 for several episodes before deciding to quit.

Blake was heralded as an expert crab fisherman. He was 38.

RIP