I just created an account in the forums when i read about eternal items cant have a cyan affix… Please check my bow, i put it at premium stash 2… It had a cyan affix on it… Deadly strike…
Im not sure where i got it but i just kept it coz i havent seen an eternal with a cyan affix on it.
It is not a bug.It is very lucky drop!
@Dough an Eternal Item can be found with a Crystal (Cyan) affix, if the affix is in the Crystal affix list. it is rare, but there are a few ways to increase the chance to find items with the Cyan affix, but it just makes them a little less rare. 
Obsidian is the Crystal used to put the Cyan affix on an item, but it is one of the Crystals that can’t be used on Eternal items.
When you get a chrystalline set item then it becomes less rare
the Crystaline Set (Green) does increase the chance for items to have a Crystal Affix on them. only 2 items have them, and they can only be found as loot, can not be rolled with Crystal (Ruby). usually used as part of a farming build.