There are Mag’har Orcs in a few places in Outland, one of them being in Hellfire.
But that’s not the case. Because Thrall, who never drank the blood, was still subject to the curse even after the Frostwolves abandoned the Horde to move to Alterac.
Generations of Orcs today, still being born with green skin even after Mannoroth’s death by Grom Hellscream. Orcs not even exposed to fel magic. Their skin should revert back to brown, yet it doesn’t.