theme customizer – can a single option pass multiple values?

I understand what you are trying to do, but you are going in the wrong direction here, you can just check if one was selected in your logic and assign the value there like this:

function css_customizer(){
  $blog_setting_val = get_theme_mod('blog_setting');
  $opacity_val = ($blog_setting_val == 'block') ? '1' : '0';
?>

<style type="text/css">
    .blog__Link{display: <?php echo $blog_setting_val; ?>}
    .second_css_class{opacity: <?php echo $opacity_val; ?>}
</style>
<?php
}

add_action('wp_head', 'css_customizer');

that should do it for your particular case.