As Psomas, correctly, said, Blogger is too strict with changes. I haven't been working with it for ages. Just because I hate have constrains when we speak about the web. Wordpress is more open and customizable. Anyway to the point, some things that might help you out.
- Check page's encoding setting. Usually (HTML5 standards) in the head you should find: <meta charset="utf-8"> just in top of the title tag (it doesn't really matter actually but the best use is just below the <head> start tag.
- Check your blogger settings. I suppose you can change the language of your blog to support Greek language posts and show them properly.
- The ISO-XXXX is something old and imo should be avoided. I know it's for Greek language but you should probably use UTF-8 for best experience.
- You can always switch the encoding of the page by the 'View' -> 'Character Encoding' option in the browser (location might differ from browser to browser) and check whether is that the fault or something else.
Report back here, if you still have issues.
Blane