How can I programmatically set a default select option in for loop?

Question

I have a for loop in my code that creates elements in a table using data from the database. Each row has a select element, where the default option should correspond to what's stored in the database:

$listmode = [
    '#type' => 'select',
    '#options' => [
        'none' => t('Unlisted'),
        'black' => t('Blacklist'),
        'white' => t('Whitelist'),
    ],
    '#attributes' => ['id' => ''],
    '#default_value' => '',
];

$rows = array();
foreach (BlocceConfigStorage::getBlocks() as $block){
    //I really wish there were a better way to do this
    if ($block->listing == null || $block->listing == 2){
        $default = 'none'; //set select to unlisted
    }
    else if ($block->listing == 0){
        $default = 'black'; //set select to blacklist
    }
    else if ($block->listing == 1){
        $default = 'white'; //set select to whitelist
    }
    $listmode['#default_value'] = $default;
    $pre = $listmode;
    $pre['#attributes']['id'] = 'select-block-'. $block->id;
    $tlist = drupal_render($pre);
    $rows[] = [
            'data' => [$block->id, $block->info, $tlist]
        ];
}

I attach the generated rows here:

$form['table'] = array(
    '#type' => 'table',
    '#header' => $thead,
    '#rows' => $rows,
    '#attributes' => array(
        'id' => 'blocce_blocks',
        ),
    );

I've tried messing around with the sample found here and adding a single listmode select to the form and setting the default. Both of them worked on their own; I've also tried pre['#default_value'] = $default, but that also was availed me nothing. I suspect that the problem either lies in the drupal_render($pre); (without this, however, the selects refuse to show up) or that the for loop itself is the problem.

Tl;dr: How do I set the default select option in the for loop?

Edit: I'm using Drupal 8, but from what I've seen, this doesn't seem to have changed much from Drupal 7.