This is one of the things that got me when I first started using Roots - there is no archive.php file, so when you’re looking at your blog index, it’s falling back to index.php.
Index.php calls from templates/content.php, so if you want to edit the_excerpt (or the_content) you change it there.
It got me a little twisted because the individual single.php template files has the full loop, but the index.php calls content.php. So that’s probably what you need to check to make this work.