>>17971852 (OP)
Drinking from copper/silver/gold ware is why the blue-skin gods really had blue skin.
Later on we have conflated it with a depiction of "dead" skin, but the earlier royals would have drinked from the esrilest batches of mined ore from furnaces with lower temperatures than the later ones, meaning silver impurities would leach out over time and turning their skin blue.
Also they had access to the metal in large enough quantities for their skin-contact and sheer amount of silverware-stored foods to make a difference.
I wouldn't be surprised if someone, somewhere, invented their case of tattoo usage as an initial attempt at replicating blue skin, before eventually tattooing symbols and figures instead of tattooing all of it blue.