How to specify the correct private file path in a block
drupal will not let you specify a direct link to a private file 

If you upload the file using file field (core) then you can use views to display the file in a block using block display and use https://www.drupal.org/project/private_files_download_permission for fine grained permissions on who can download the file

if you want to program this behaviour then you have to look at using file_create_url in a custom module

$fid = 1; //set fid to your file's fid.
$downloadlink = file_create_url(file_load($fid)-&gt;uri);


and this link will hide the actual path where you private file is

for more details you can look at

How can I get the URL to the public files directory?

https://api.drupal.org/api/drupal/core%21includes%21file.inc/function/file_create_url/8.2.x