You have to prepare you xls columns as below:

name: (Mandatory) is the title of the blog post

blog_id: (Mandatory) Name of the blog and it should be created first in the system or you can create it during import.

subtitle: The post subtitle if any or leave it empty.

author_id: The author of the post if any and it should be create in the system as partner or you can create it during import. You can leave it empty.

tag_ids: The column will have your post tags and if you have multiple you will separate by comma. (tags should be created before upload or you can create them during the upload as well)

Content: You post body, you can put your post even with HTML tags.