Not every theme displays a Tagline. A Tagline (not a blog description) is meant to be a very brief branding message. Your expanded blog description is meant to appear on your About page.
The Site title (blog name) and/or the optional Tagline can be changed on the top of this page at any time. > Settings > General … “save changes”.
That is exactly where search engine spiders look for the site title and removing it from there can cause issues with getting your content indexed. Note that the site title (blog name) and blog address URL do not have to be the same but it’s better for SEO (search engine optimization) if they are the same.