Please copy again the code and replace with the code you have. Save it in the functions.php file of your WordPress theme, then delete all caches and check it.

Also, I want to see a screenshot of the code in your functions.php editor. And other codes in that file.

Yes, it's a good idea. We'll probably discuss this.

So, is it fixed?

If I replace "body" in the original code with "blockquote" it works perfectly for the blockquote, but getting both to work either crashes tinymce or does nothing for the second parameter.

You should add CSS for blockquote in the CSS part ($settings) of the code I provided before:

 blockquote { background: #3C3C3C !important; color:#dddddd !important; }

So you can remove the code for background and put this one, the final code should look like this:

function wpforo_custom_editor_css( $settings ){
    if( isset($settings) ){
        $settings .= ' body{background-color: #000000!important; color: #fff3ca!important;} blockquote { background: #3C3C3C !important; color:#dddddd !important; }';
    }
    return $settings;
}
add_filter('wpforo_editor_settings', 'wpforo_custom_editor_css', 12);

Thanks