Talk:Paxton McCreary/@comment-184.68.184.98-20180717222352/@comment-27794543-20180910085639

Eligius III (the people who went to the two sun planet) were given Nightblood. Eligius IV (the prisoners seen in s5) have regular blood, as seen by them bleeding red. This was also confirmed by Shaw when he saw Clarke bleed black.

But back to the original question, this was never explicitly explained in the series. Like the other IP said, it's possible that their exposure to solar radiation (being in space for a century) caused them to build up a certain immunity to radiation allowing them to withstand the leftover atmospheric radiation on Earth.