Register a Custom Gutenberg Block Category

add_filter( 'block_categories_all', 'wpfolks_custom_block_category', 10, 2 );
function wpfolks_custom_block_category( $categories, $block_editor_context ) {
    return array_merge(
        [
            [
                'slug'  => 'my-theme-blocks',
                'title' => __( 'My Theme Blocks', 'textdomain' ),
                'icon'  => 'layout', // Any Dashicons slug
            ],
        ],
        $categories
    );
}