block--[region].tpl.php not working I Copied the file

bootstrap\theme\system\block.tpl.php


to my subtheme

bootstrap_subtheme\templates\block.tpl.php


I made another 2 copies and renamed them to

bootstrap_subtheme\templates\block--sidebar-first.tpl.php
bootstrap_subtheme\templates\block--sidebar_first.tpl.php


In the subtheme.info file it says
regions[sidebar_first]  = 'Primary'

The block.tpl override works but the sidebar-first.tpl.php does not get overridden.

What am I doing wrong?
The name of your template block--sidebar_first.tpl.php is correct. Clear the cache to override the template.
OR 
Put this function in template.php and clear the cache. You will get the suggested template for your block.

On suggested name replace double underscore with double dash for your tpl file, and clear the cache.

function THEMENAME_preprocess_block(&amp;$variables) {
    print_r($variables['theme_hook_suggestions']);     
}