grabbing data and displaying it in block twig bI made a custom module and made a custom block inside of it. 

In the build () I have this. 

return array (
  '#theme' =&gt; 'no_athlete_match_block',
  '#title' =&gt; 'Athletes With No Matching Name in the Database',
  '#description' =&gt; 'Displays the athletes with no matching name in the database for a given sporting event.',
);


In the .module file I have this. 

function no_athlete_match_theme($existing, $type, $theme, $path) {
  return array(
    'no_athlete_match_block' =&gt; array(
      'variables' =&gt; array(
        'description' =&gt; NULL,
        'title' =&gt; NULL
      ),
      'template' =&gt; 'block--no_athlete_match',
    ),
  );
}


my twig file named block--no-athlete-match.html.twig

&lt;div&gt;
node: {{title}} {{description}}




Only Node: shows up. If I do dump(title) I get NULL to show up. Not sure what I'm missing. I've copied other blocks and still nothing.
Because you've specified the template parameter in the hook_theme function the template file name should be exactly as specified block--no_athelete_match.html.twig. The underscores are converted to dashes only in the case where the template parameter is not specified in the hook_theme function.