You might want to customize the code for your blog posts. In the site editor, go to Settings > Blog > Blog Settings. Scroll down, and you'll see the "Header Code" and "Footer Code" boxes.
Header code will be added in the <head> tag of each blog post. Footer code will be added before the closing </body> tag of each blog post.
Remember, this custom code is only applied to blog posts, not your main site pages. Likewise, the custom code setting on your main site is not applied to these blog posts. For example, this is useful if you want to modify the styles of your blog posts.
NOTE: Custom code is available for the Pro plan and above.