It seems to be a new thing that Blizzard figured out they could do. Many Allied Races as well as Demon Hunters have the option for body tattoos now. As well as certain other features being shared, like the hair effects on Dark Irons and Void Elves...
Considering this and looking at Alleria's model with her blue tattoos, she really just looks like a normal High Elf with blue tattoos, so wouldn't that make it perfect for a High Elf Allied Race? Pretty High Elves with blue eyes and blue tattoos? Seems like they would be very distinct from all the other Elven races and almost looks like they were planned to be an Allied Race at some point. The tattoo thing just seems like it can't be a coincidence.