As stated in the documentation (https://doc.ezplatform.com/en/2.3/tutorials/enterprise_beginner/3_use_existing_blocks/#add-a-content-list-block) it should be possible to override the default block template as well as other settings.

Steps to reproduce

1. Create own template for one of the blocks (for instance Collection). Place it in app/Resources/blocks/.
2. Configure block as the following:

   ezplatform_page_fieldtype:
       blocks:
           collection:
               name: Own collection
               category: default
               thumbnail: '/bundles/ezplatformadminui/img/ez-icons.svg#collection'
               views:
                   default:
                       template: ':blocks:contentlist.html.twig'
               attributes:
                   own:
                       type: text
   

3. Clear the cache.
4. Open PageBuilder and add block.

Result
After adding block the default template is still in use. What is worth to notice, only attributes are taken from the custom configuration, all others like name, thumbnail still come from default settings.