Now i found this explanation:
Register a blocks directory.
If a namespace is provided for the render, Twill will assume it to be under: NAMESPACE::site.blocks.BLOCK-NAME
Given $this->loadViewsFrom(__DIR__ . '/../resources/views/site/blocks', 'package-name');
So if you have a block called “example” and you want your package to provide the preview. Your file should be in resources\views\site\blocks\example.blade.php.
To make sure the end user can override the views, you should make them publishable:
$this->publishes([
__DIR__ . '/../resources/views' => resource_path('views/vendor/PACKAGE-NAME'),
]);
For me (__DIR__ . '/../resources/views/site/blocks' would be (__DIR__ . '/../resources/views/twill/blocks' for the blade files to add the fields to the singleton in the cms. But the blocks editor is not showing.