Custom module over ride blazy.html.twig template
I found the issue to override the module template you have to call the modules name in the _theme hook $return['blazy'].

function slider_overlay_text_theme($existing, $type, $theme, $path)
{
    $templates = $path . '/templates';

    $return['blazy'] = [
        'template' =&gt; 'blazy',
        'path' =&gt; $templates
    ];
    return $return;
}