you wash your mouth you pray for truth

The way Sean looks at me has slowly changed. It’s not as aggressive as before. The wall he built is gradually breaking. The secret he told is like a door I can walk through to get closer to him.

i was calm before but now i’m wondering why my heart is fluttering when we meet / what is happening inside, i’m shaken seeing you every day has become different

Everything I say comes with the caveat that I haven't read the novel so I don't know how he's characterized in that and in the show Black isn't the most developed character. This isn't a criticism (Black serves a vital narrative role by being the catalyst for the entire story), but it does mean that most of the information about Black is inferred by the audience rather than witnessed.

I've seen some people disappointed that Black doesn't stay with the gang at the end of the show. I agree with this to some extent because I love the found family trope and I just want everyone to be safe and happy and loved. But from a narrative and characterization stand point, it makes total sense to me that Black wouldn't want to stick around.

For most of the show, the gang is not a Found Family. They are at best a group of friendly acquaintances united in a common purpose. There are friendships within the group--Sean & Yok, Yok & Gram, Gram & Black--but the whole group doesn't gel. We can infer from the first couple episodes that the reason the group doesn't gel is because Black is openly hostile to everyone. Sean and Yok especially are suspicious and confused when White interacts with them and isn't a huge asshole. But we also learn that Black is the defacto leader of the group because he's the most driven, the most reckless, and the most unwilling to collaborate. Black brought everyone together, but he's not interested in making friends. He's here to watch the world burn for justice and he doesn't need to play well with others in order to do that. For all that he believes in social justice, he has a more authoritarian approach to working with others.

It's no surprise that that approach isn't going to be sustainable in a group. Most people can't go through the life like that. That's what Gumpa meant when he said that White was better than Black. White is a diplomat, not a dictator. The tension he has with the group, and Sean in particular, is because in the beginning everyone is expecting him to behave like Black. @dyeyourhandsblue has some great meta that goes in depth about Sean and White's interactions and how White is constantly undermining Sean's expectations in a good way. The group begins to feel more like a Found Family, and work more cohesively, because White cares about their survival. White can see the bigger picture which shows that a functioning group that works together can create more change for longer than a dysfunctional group hellbent on martyring themselves for their ideals. White's presence changes the dynamic of the entire group, which is shown most dramatically in how Sean changes. Sean was begrudgingly content to follow Black because neither of them have self-preservation and both are willing to use excessive violence to enact social justice. But once White shows Sean an alternative, Sean begins to value his own life and sees the benefit of less violent forms of protest.

So, at the end of Not Me, the gang is beginning to feel more like a Found Family. But Black was in a coma for all that development. Black has not changed the way the rest of the group has. And Black's not stupid. He knows that the group has changed without him and that change is probably for the better even if it doesn't include him.

I think of Black as almost like the John Wayne archetype...or like Mad Max or something. He cares about creating a just world, but if the world was just he wouldn't be able to function in it. He wants to save the village from bandits, but once the bandits are gone, he can't coexist with the villagers. He has to move on. He sees that the gang is better without his violent, domineering presence, so he takes off.

In fact, I really thought that’s where they were going: Black and Sean were constructed as so similar to each other that, just like White made Sean change and become more willing to work in a group, I really thought Black would go through the same development where White softens him up too. And White does soften him up a little bit, but since Black only shows up at the end of the show, it still wasn’t enough to not make him clash with the rest of the gang. So of course he leaves.