Twig Template for a Custom Block Plugin
This issue I ran into was I did not specify '#theme' in the Block Plugin.

I added

public function build() {
    return array(
        '#theme' =&gt; 'myplugin',
        '#title' =&gt; 'Test',
        '#color' =&gt; $this-&gt;configuration['color'],
        );
}


and it worked