To configure Blog XML sitemap in Magento 2 please, first of all, make sure that Magento default XML sitemap has been configured properly.

Once you have configured Magento XML sitemap for products, categories, CMS pages, blog sitemap will be generated automatically and will be available by the same path as your default XML sitemap but with "blog_" prefix.

Example 1

Default XML Sitemap URL:  https://domain.com/sitemap.xml
Blog XML Sitemap URL:  https://domain.com/blog_sitemap.xml

Example 2

Default XML Sitemap URL:  https://domain.com/pub/en_sitemap.xml
Blog XML Sitemap URL:  https://domain.com/pub/blog_en_sitemap.xml

If you use Blog Plus or Blog Extra editions of the Blog extension you are able to change the priority and frequency of the blog items in the XML sitemap. In order to do this please go to Stores > Configuration > Blog and find the XML Sitemap Section. Here you can manage the frequency and priority of the Blog Index Page, Blog Categories and Blog Posts.

Magento 2 Blog XML Sitemap Configuration, Blog Index Page
Magento 2 Blog XML Sitemap Configuration, Blog Categories
Magento 2 Blog XML Sitemap Configuration, Blog Posts

Blog XML Sitemap Troubleshooting

1. Why blog_sitemap.xml is missing in my Magento 2?

 - Please make sure that you use the latest version of the blog extension. Check the changelog of the extension to see if there where any sitemap bug fixes or improvements.