When the blood stops flowing, gravity takes over. The Latin term is livor mortis or blue death. Blood settles in the lowest points of the body, which depends on the position the body is in at the time of death.
If someone dies in bed, the purple streaks on their backs—similar in color to bruises—will follow the wrinkles in the sheets and show that blood hasn’t been circulating for quite a while.
Lividity can show up in as little as 15 minutes.