Although there is an "export" option via Blogger settings page but am sharing with you in this post, how to use Google Takeout to back up your blog post (including drafts) and comments. At Google Takeout, you can easily download the blog content of all the blogs in your Blogger dashboard. You can import the processed file into another blog e.g when merging two blogspot blogs together.
How To Get Started
==> Go to www.google.com/takeout and log in with the same Google account which you are using for your blogger blog.
==> Click on "Choose services" tab
==> Click on "Blogger" tab
==> Click on "Configure"
==> Select your desired back up option
If you want to back up a single blog, select "download a single blog" option. You will see the list of your blogs. Click inside the box and select the blog you want to back up.
==> Click the "create archive" button
==> Watch the progress bar. Once completed, click the "download" button
==> Verify your password if prompted to do so
==> Save the zip file to your computer.
Remember, your data is important! Do not download archives on public computers. If you use a public computer, ensure you attach and send to your mail. Then, delete it completely from the computer, even from the recycle bin.
I hope it works for you.
To back up ONLY the look and design of your blog, read how to download/backup blogspot template.