How do I override the title with the username?
Make Fields your format



Alternatively: if you are using the rendered entity row plugin check the Force using fields in the Format | Settings.
In RELATIONSHIP, Add Content author


Checkmark ☑ Require relationship

In FIELDS, add Name


Checkmark ☑ Exclude from display
Uncheck ☐ Link to the user 

For TITLE put {{ name }}'s blog



OR you could put it in your CONTEXTUAL FILTER; even though {{ name }} is not listed as a replacement pattern, it works (as long as you do step 1, 2, and 3).




Result



BONUS

If you want to capitalize the first letter of the user name use {{ name|capitalize }}'s blog.

You can also add a Rendered Entity as a field.