I got two reviews on the last chapter about the breathplay in seeming a lot more like abuse than play. And you're completely right. Klaine's relationship at this point is definitely not a healthy D/s relationship and it does not follow the rules of Safe, Sane, and Consensual. Blaine didn't want the breathplay and they didn't discuss it at all before it happened giving Blaine no way to safe word or tell Kurt that it really was too much, something you should always do before trying out hard kinks, especially ones like breathplay that could end in death if you're not careful and don't know what you're doing. Blaine doesn't really trust Kurt, it's just that at this point, he knows that if Kurt's going to continue to do things like this, he's got to make himself believe that Kurt won't actually hurt him if he's going to deal with it. He doesn't want to be terrified for the rest of his life. His decision to trust Kurt with his body is a coping method, it's how he's dealing with his slavery.
That being said, I'm not going to be exploring many hard kinks in this fic. I prefer to leave that to relationships that are completely consensual (Like my Icy Love verse). This scene was not about depicting a sweet, intimate moment of a sub choosing to give up control over literally everything to their Dom, it was Kurt forcing Blaine into his slave headspace, Blaine is new to this afterall. I wanted to include this, with Blaine forcing himself to trust Kurt because that trust fantasy (I guess) is going to come into play later in the fic. I won't talk much on it, as it contains a lot of spoilers, but just know, this more abusive side of their relationship is only going to happen once more, everything else will be sweet and gentle between them with minimal BDSM, mostly just the Master/pet dynamic, Kurt ordering Blaine around, and maybe some more bondage at most. Kurt's not a sadist, he doesn't really want to hurt Blaine, he just wants him to know that he's his and that's not changing.
And on the topic of Blaine saying him and Kurt are friends, and we mostly know of the show, Blaine has a huge heart and generally forgets about his own pain if he sees something wrong with someone else, even if said person is the cause of his pain. It made sense to me that Blaine would want to soothe Kurt's loneliness as best he could, and let him know that he could be considered a friend. Kurt's reaction to his comment will be further explored in the next chapter.
I hoped this helped and I hope you two reviewers will continue to read!