In order to create a new tag in Magento 2 Blog navigate to Content > Blog > Tags and press Add New Tag button.

New Tag

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.

Magento 2 new tag

3. Set the URL Key and Meta Information for the tag. Choose the Meta Robots you want to be enabled on the tag page.

Magento 2 tag search engine optimization

4. Set the Design and Custom Design Update.

Magento 2 tag design

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.

Magento 2 blog demo

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.

Magento 2 blog post page

After you save the changes you will be able to navigate to posts under the corresponding tag.

Magento 2 tag page demo