How to add a custom field to views block
Views is used to pull in existing data/content from other nodes so that you can customise and display it either in new a page or block. There is no body field on the blocks page for a views block is because the contents of that block is controlled by the view and can change dynamically. So if you want to change the body text displayed in the view you can edit the body field of one of the  content nodes your view is collecting from.

When I want to add extra custom text to a view I usually add a random field to the view and then inside that fields settings enable rewriting to allow rewrite the fields output with custom values, if you want to go a bit deeper you can use the module Views Custom Fields as another alternative: https://www.drupal.org/project/views_customfield, but as far as I know editing the body of a views block directly from the blocks admin page is not possible without writing some custom programming.