Spoilers ahead for Episode 10 of The 100 Season 7, called "Etherea."
The 100 finally went ahead and confirmed that Bellamy Blake is very much alive with "Etherea," but the episode was far from a straightforward reveal that Bob Morley taking a break didn't have to mean Bellamy biting the dust. Bellamy and the Conductor of the Anomaly Stone on Bardo were knocked through the bridge to the planet Etherea rather than vaporized in front of Octavia's eyes, and they were gone long enough that Bellamy went through a big change, and the result was a huge betrayal that could change everything for Clarke.
Thanks to time dilation, Bellamy was gone for long enough to grow a full beard, long hair, and a belief system that turned him into a follower of the Shepherd. Bellamy had no choice but to join forces with the Conductor on Etherea to try and climb a frozen mountain to reach an Anomaly Stone that could take them back to Bardo, and the process converted him.
With no company other than the Conductor, events that seemed to support his preachings about the Shepherd, and an experience in a cave involving the hallucination/manifestation of his mom (possibly fueled by living in a freezing cave and eating nothing but wall lichen), Bellamy was convinced that the Conductor's faith in the Shepherd was rewarded.
When they returned to Bardo, Bellamy's friends were understandably thrilled to see him, despite good old Bell not exactly looking his best. They were less thrilled when he almost immediately betrayed them. Desperate to continue the ruse that Clarke has the Flame in her head, Clarke threw her arms around Bellamy to hug him... and clue him in on the big secret, telling him to "say nothing."
Bellamy did exactly the opposite, and he turned to Bill Cadogan to reveal that "the Key" has been destroyed and that Clarke has nothing. The episode ended before Clarke and Co. could do more than look shocked that Bellamy said "My Shepherd" and then followed by spilling the only secret that was keeping them all safe, but the result undoubtedly can't be anything good. The big question is: how will Clarke react?
Bellamy's betrayal could have a major impact on Clarke in two ways: personally and as leader of their ragtag group of friends. Bellamy and Clarke spent six seasons sharing leadership duties and becoming best of friends despite the occasional massive fight and abandonment, and he has more or less been her person that she can count on. Bellamy didn't even seem conflicted about telling Cadogan that Clarke didn't have the Key, which could mean that Clarke the leader lost her greatest ally in saving the day, and Clark the woman will have to deal with the emotional fallout of her best friend's betrayal.
Basically, Bellamy betraying Clarke and the group may have ruined their hopes of escape and survival. There's no telling how Bill Cadogan will respond to the news that the Key is gone and Clarke has been deceiving him. From an emotional standpoint, the odds are pretty good that Clarke, Octavia, and especially Echo won't respond too well. Echo has gone from one extreme to the other dealing with the loss and then "death" of Bellamy; will she be able to cope with this unexpected twist?
Of course, the biggest question now is whether or not the Bellamy from Seasons 1-6 is really gone and replaced by Disciple Bellamy. Despite his love of mythology and history, Bellamy never showed especially religious inclinations before this, and it's possible that being around people in a safe place rather than in the wild with just the Conductor will bring him back to who he was.
Bellamy also has plenty of experience with infiltrating enemy ranks and causing trouble from within, so he could be incredibly valuable if Clarke and Co. find a way to turn him back. Hey, if a few months on Etherea could turn him into a follower of the Shepherd, couldn't his friends citing six seasons and 100+ years bring him around? Jordan has his doubts about the way things work on Bardo, so perhaps he'll have a part to play. Then again, we're in the second half of the final season of The 100, so we have to consider that maybe Bellamy as we knew him is gone for good.