Meta description for both blog and post for Blogger is disabled by default. It is good for SEO as well as increasing the chance that people searching for something related to your blog or posts will click on the search results to get to your blog and posts.
With this default set, when you compose a post, you will not see an option to enter search description for a post and typically this screenshot would show the missing field for search description in the right sidebar of the post editor:
You will see a change when you enable meta description for your blog as shown below.
When you sign into www.blogger.com
Dashboard > Settings > Search preferences
you will see that meta tag for description for the blog is disabled:
Click EDIT, the tick ENABLE SEARCH DESCRIPTIONS? (.) Yes, then enter a suitable search decription for the blog then click SAVE
Now when you open the post editor to compose a post, you will notice a difference in the right sidebar. A new section for SEARCH DESCRIPTION will be added for you to enter a suitable search description for the post (see screenshot below):