<?php
/**
I think this sets block attribute from get_option() I don't see how to get value from DOM to update_option() Is there a pre save hook for block attrs?

I didn't test this!
*/
$block = new WP_Block_Type('zg/block',
    [
        'attributes' => [
            'hi' => [
                'type' => 'string',
                'default' => get_option( 'hi', 'roy' )
            ]
        ]
    ]
);
WP_Block_Type_Registry::get_instance()->register($block );