Asuka, are you able to obtain any meds by Seachem or API or is it just Sera meds?
If you can find meds by Seachem or API, look for Seachem Metro or API General Cure. If not, I'm unfamiliar with most Sera meds so look for a med that has metronidazole in it. That will treat what I think is almost certainly an internal parasite infection.
If Blueberry is an older betta, his ability to fight off infections will be reduced but it's still possible. He can still beat this. Either way, it certainly won't be because you didn't try. You're doing a fantastic job giving Blueberry and your other bettas such devoted care, Asuka.