>>531558195
>>531558449
A few had general Sha corruption with Garrosh making them labor away getting the Dark Heart, yeah. One of the scenarios has you fight a fully corrupted one who changed colour.
But I mean also just look at the machines they were working on at the time. The Iron Juggernaut, the mech Siegecrafter Blackfuse was making, all of the siege weaponry they brought to the Iron Horde, the entire train system the Blackrock Orcs had, and the Iron Reaver after they got a hit of Fel from Gul'dan.
There's A LOT of fire and steel they're having to put together for all those weapons, it could also just be easily implied they're covered in soot and ash. Dirty goblins!
Sidenote, soot/ash/grease faces were a customization option I've wanted for Goblins and Gnomes for fucking years.