Hi Melissa,
My understanding is that currently there is no notification system for blog comments that will email all previous commenters. (unlike this forum, which does have one). For Shopify blogs, replying to comments requires an app.
Apps like Sunny will notify the original commenter when you respond, but not other commenters. Disqus is another app for comments that may have more features.
I hope that helps,
Jon
(Sunny is here: https://apps.shopify.com/blog-comment-manager. Disclosure; I built this 
).