How do I make it so that the default choice is blank (i.e. no time chosen by default)?

You can do this using the openinghours_frontend_initialvalue filter in an mu-plugin.

i.e.

Place a file in wp-content/mu-plugins/no-initial-value.php (you may need to create the mu-plugins directory if it does not yet exist):

<?php
add_filter('openinghours_frontend_initialvalue', '__return_empty_string');
add_filter('openinghours_frontend_initialvalue_formethod', '__return_empty_string');

Posted in: WooCommerce Opening Hours and Delivery Times