The blue unread bar drives me absolutely NUTS!!! Here's the only way I have found to get rid of it: (Win 10, MS 365)
As stated above, change View to "Single" , THEN select View Settings-> Group By -> Uncheck "Automatically group according to arrangement" -> Then choose "(none)" under "Group items by". This made my blue bar go away. Hope this helps...
Now if they would just fix the ugly washed-out color pallet in categories... This is a mail management system, not a Spring fashion show!! (*audible eye roll*)