Doctor Who Series 10: 11 Easter Eggs & References In 'Oxygen'
7. Blue Aliens
One of the crewmembers of Chasm Forge is the blue-skinned Dahh-Ren (don't call him Darren).
The character is the latest of many blue-skinned aliens the Doctor has encountered over the past ten seasons of the modern series. Back in 2005's The End of the World, for example, the crew of Platform One, the space station observing the death of the Earth, were blue people from the planet Crespallion. Later examples include Dorium Maldovar, the proprietor of the Mos Eisley Cantina-like Maldovarium and occasionally ally to the Doctor.
Interestingly, Dahh-Ren has yellow eyes, unlike Dorium but identical to the blue people of The End of the World. It is presumably the intention, then, that he is also a Crespallion.