To configure Magento 2 Blog XML sitemap 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.
Default XML Sitemap URL: https://domain.com/sitemap.xml
Blog XML Sitemap URL: https://domain.com/blog_sitemap.xml
Default XML Sitemap URL: https://domain.com/pub/en_sitemap.xml
Blog XML Sitemap URL: https://domain.com/pub/blog_en_sitemap.xml
Note that you will not see any additional sitemap item in the sitemap grid in the admin panel. Blog sitemap XML will be generated automatically in the background together with the regular sitemap.
Once a blog sitemap is generated we strongly recommend adding its link to the robots.txt as it is done at Magefan (https://magefan.com/robots.txt).