In order to create a new tag in Magento 2 Blog navigate to Content > Blog > Tags and press Add New Tag button.
1.Make sure you Enable the new Tag.
2. Set the Tag Title and write some Content that will be displayed on the tag page.
3. Set the URL Key and Meta Information for the tag. Choose the Meta Robots you want to be enabled on the tag page.
4. Set the Design and Custom Design Update.
Once you Save the new tag you will see it appear in the tag cloud widget on the storefront. Read more to learn about how to configure the tag cloud.
In order to assign a tag to a blog post go to Blog Post Page ( Content > Blog >Posts ) and in the Tags field set the Tag you want the post to be found by.
After you save the changes you will be able to navigate to posts under the corresponding tag.