little is available in template-page-blog.php to be edited manually as thematic does everything by filter and action hooks. if you follow the thematic_indexloop() hook into the extensions folder, you'll see the thematic_index_loop is added to that loop. within that function you'll find thematic_content() which is the function that determines how excerpts and full posts are displayed. this allows for crazy-high levels of granular control, but is also confusing to new people.
1. why are you using the blog template? can't you just set your Reading Options? I ask, b/c we're discussing deprecating this template.
2. once you've found thematic_content you'll see that you can filter the thumbnails off completely.