register_block_type – no block available in block editor

Fresh/clean install of WordPress 6.1.1 (no 3rd party plugins/themes)- PHP 8.0.x

Using the following code in functions.php of twentytwentytwo theme.

function wrm_testblock() {
  register_block_type('wrm/testblock', array(
    'attributes' => array(
      'title' => array(
        'type' => 'string',
        'default' => 'TEST',
      ),
      'imageUrl' => array(
        'type' => 'string',
        'default' => '',
      ),
      'linkUrl' => array(
        'type' => 'string',
        'default' => '',
      ),
    ),
  ));
}
add_action('init', 'wrm_testblock');

According to any documentation and other examples I have read, I would expect to see the new block available in the blocks panel in the editor.
But that’s not the case. The new block is nowhere.
If I do intentionally a mistake -e.g. type the block name with capitals, WP will let me know about my mistake, which means that the function gets called.

I might be missing something obvious that I am not able to see.

Does the block need to have any other attributes in order to become available in the blocks panel? Or do I need anything else to make this work?

All the docs I read and many examples show that I can register a block only by its name and that it should appear as block option in the panel.

Any idea?