However, prior to that, TV Guide has it from EP Jeffrey Bell that the blue alien race is indeed - as pretty much expected - the Kree.
"Our people don't know anything about the Kree or that there's a planet full of them," Executive producer Jeffrey Bell said. "What they do know is that the strange carvings created by Coulson after he was injected with the Kree serum are actually the map of a city, and they need to find that city before Hydra does. But where is it? Here or on another planet?"
Both Coulson and Skye have been injected with the Kree serum, with the latter having the side effects leading to the mysterious city. Current speculation is that the city is Attilan, the home of the Inhumans, which is located on the moon in the Marvel Comics (an Inhumans movie was also recently announced by Marvel Studios). It's also speculated that Skye may be an Inhuman, which Bell may be hinting at.
"You'll find out that Skye and Raina have something in common — and it's not that they're brunettes [laughs]," Bennet said. "Some of our überfans think the two of them are aliens. Or maybe they're something else — something different and special. It's all leading to a reveal that's mother-effing crazy."
"Marvel's Agents Of S.H.I.E.L.D." airs Tuesdays at 9pm ET on ABC.