It may sound harsh, but the characters are just characters and you can redo them - and it will probably help.
I don't make a comic, but I have a partly written-mostly discussed universe (together with other people), where one of the three main characters is my ex best friend's character. And all other OCs that are linked to this character play a pretty huge role in the plot too.
After a quarrel, all I had to do is to own that character now, so I did my best in making her my character. She didn't change in the core, but overall I tried to change and evolve her and all the associated characters in the way I'd like them to be. So I gave them new plotlines, new hobbies, clothes and features.
If it was just a cameo and they didn't wrote you anything about it - leave it be, if it doesn't bother you much. If it bothers you - redraw it.
If you want to apologize and write them something - please do, get this off your chest. But there's nothing you can do if they don't want to talk to you. If they blocked you, they are probably angry and don't want to have anything to do with you for now. So just own your comic, make the changes you need to make for it to feel like your own.