It might look like it's blocking your actual blog posts but remember that Shopify's structure also includes the name of the blog. For example, many people have theirs like /blogs/news/articles so in this case, the robots.txt is blocking the /blogs/news archive page. So the default file won't stop your blogs from being crawled (Shopify isn't the best for SEO when compared to WordPress but they would never want to block the blog posts from being crawled/indexed haha) 
the best way to find out is using the Google robots.txt tester tool. Simply enter your file, then put the URL of the page you want to test in the bottom bar, then it'll tell you if it's allowed to crawl it or not based on your file: https://www.google.com/webmasters/tools/robots-testing-tool?utm_source=support.google.com/webmasters/&utm_medium=referral&utm_campaign=%206062598