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');
add_filter('openinghours_frontend_initialvalue_formethod_raw', '__return_empty_string');
add_action('wp_footer', function() {
?>
<script>
jQuery(document.body).on('openinghours_initialised', function() {
jQuery('#openinghours_time').val('');
});
</script>
<?php
});

Posted in: WooCommerce Opening Hours and Delivery Times