Add the script once in your site settings, then a Code Embed on your blog post template.
Sign in to Crowment, create a comment widget for your site's domain and copy its app key from Install & Embed.
In Site settings → Custom code → Head code, paste:
<script defer src="https://embeds.crowment.com/comments.js"></script>
Open your blog's CMS template page (for example Blog Posts Template), drag a Code Embed element below the post body and paste:
<crow-comments app-key="YOUR_APP_KEY"></crow-comments>
Publish the site and open a post. The Designer doesn't run custom code, so the comment section only shows on the published site.