Given a properly developed blog, you can add meta tags, on a per blog, per page, and per post basis. The tags can indicate, to your prospective readers, what the blog - and each page and post - is about.
You add meta tags, describing the blog, and the individual pages and posts, using the Blogger dashboard wizards.
Add a blog description, before you add the page and post descriptions.
Add the description for the blog, first.
Start with the description for the blog. Use the "Meta tags" option in the dashboard page Settings - Search preferences. Edit the Description, and enter the blog description.
What Blogger won't (or can't) tell you. Blogger features and problems explained, using real life examples.
Then add the page and post descriptions, one page or post at a time.
Once the blog description is entered, the page editor "Page settings" and post editor "Post settings" menus will have a new option - "Search Description". Open that, and add the search description, for each page and post, one by one.
Then add each page and post description, one by one.
Template code needs proper references to search description.
With some meta tags, there is a discrete and separate pair of tags - there will be one tag that contains the per blog content - and a second tag that contains the individual per post content. With the Search Description, both the per blog and per post content is passed in one tag.
When you add template code, you won't need any conditionals to determine whether you are in main page or post page display - the same tag will be used for both displays.
Make all descriptions brief, and descriptive.
For both the blog, page, and and post descriptions, just enter one or two brief, descriptive phrases or sentences. Don't bother with meta keywords - they don't help, and may hurt, search reputation.
Blogger creates the XML code, from the appropriate description - and inserts it in the right location, in the template header section.
Note that you will index individual posts - and you write search descriptions for individual posts. You may, if you wish, index individual pages - and you may write search descriptions for individual pages.
You can add descriptions for post pages, and static pages.
You won't write search descriptions for archive retrievals or label searches - because neither archive retrievals nor label searches are (should be) indexed by the search engines.
Read Webmaster Tools Help: Review your page titles and snippets, for details about how to create good meta tags - and see my examples, in the screen prints, above. Use the SERP Snippet Optimization Tool, to make your search entries both compact and readable.
Write the descriptions carefully. Make each single character count.
A search description can be a maximum of 140 characters (give or take a few, and this is important, too) - you will see the concept in the Optimization Tool. Make your 140 characters count - this is what your prospective readers will see, when they must decide whether to read your blog / page / post, or another.
Allow 10 - 15 minutes to write a good, tight description, for each post. If you can spend a couple hours (or how many days?) writing a good post, you can take 10 - 15 minutes, to write a description of the post.
Description, Title, and URL get limited space, in a search entry.
Just remember that search descriptions complement content - they don't replace it. Properly developed blogs start with informative, interesting, and unique content.Learn to describe your blog, and the posts, to the search engines - and increase traffic from people viewing the search listings.
A possible alternative / complementary technique, popular with some blog owners, is to put the post title ahead of the blog title.
However you do it, don't add meta tags before you have a blog that people will read - and that search engines will index.