Thanks for comments!
Strap is finished with neatsfoot oil, narvsvärta (dye made in Sweden, or Scandinavia in generally. I think it's quite similar with other alcohol based dyes), Tandy Super shene, Sheridan antique paste and finally leather conditioner. It's not "sweatproof", but I think it's thick enough to prevent dye bleed. And also these alcohol based dyes seem to resist bleeding quite well.