Soot tattoos are a cutaneous condition that may be a sign of drug abuse, a condition produced by injections of residual carbon on the needle after flaming of the tip.
Soot tattoo Wikipedia
Soot tattoos are considered the oldest form of tattoos. The 5300 year old body of Tyrolean iceman Ötzi is decorated with simple tattoos, and researchers think they may have served a medicinal purpose, not a decorative one.
Soot tattoos are still used in some countries.