Bellissima is the code name for the rebuild of the frontend code of the backoffice of Umbraco. Bellissima replaces the old deprecated AngularJS code with Lit, TypeScript and Web Components as well as a flexible extensions API.
Joe Glombek discusses configuring custom backoffice block views in Umbraco using an extension. He provides an example of creating a hidden block indicator, detailing the registration of the extension and the creation of a custom element. The implementation allows for automatic display of a hidden state in block lists.