Pre-set CSS class into custom block/template

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In case anyone is looking for an answer to this that doesn’t involve adding a filter and a bunch of extra code, you can add a className to the block directly in the template code. This is the same code that the OP used, with one small adjustment. “class” becomes “className”:

function myplugin_register_book_post_type() {
    $args = array(
        'public' => true,
        'label'  => 'Books',
        'show_in_rest' => true,
        'template' => array(
            array( 'core/image', array(
                'align' => 'left',
            ) ),
            array( 'core/heading', array(
                'placeholder' => 'Add Author...',
                'className' => 'blockHeadStyle anotherClassName'
            ) ),
            array( 'core/paragraph', array(
                'placeholder' => 'Add Description...',
            )),
        ),
    );
    register_post_type('book', $args);
}
add_action('init', 'myplugin_register_book_post_type');

Here is a screenshot to show how this renders in dev tools:

Here is a screenshot to show how this renders in dev tools

As you can see, I also added a second class to the h2 element, simply by adding a space between each class declared by “className.”