How can I add special classes to only all blocks that's generated in certain region?

Question

I want to add some classes to each block that generated in custom region automatically.
For example I want to all blocks that generated within sidebar-left have some special classes and in addition I don't want to those blocks in other regions have those classes.

I try to use something like below code to add desired class to each block that's generated in sidebar-left:

region--sidebar-left.html.twig

{%
  set classes = [
    'region',
    'region-' ~ region,
  ]
%}
{% if content %}
  <div{{ attributes.addClass(classes) }}>
    {{ content }}
  </div>
{% endif %}

But it seems there isn't any loop functionality Like below to I can use for each blocks and add my desired classes to them:

{% for item in items %}
  <div class="my-desired-class">
    {{ content }}
  </div>
{% endif %}

Answer 1

I am afraid this is no longer possible, according to Region-specific block templates are not available in Drupal 8.

In Drupal 7 it was possible to specify separate block templates per region, such as block--header.tpl.php and block--footer.php to output different HTML for blocks placed in the header and footer regions. It was also possible to adjust variables based on region in hook_preprocess_block(). Both of these features have been removed.

It is suggested you use standard CSS cascading selectors to style blocks differently on different parts of the page.

Therefore, you have 2 options:

1) In your page.html.twig you add a parent/wrapper class or id

{% if page.sidebar %}
    <div class="my-sidebar">
       {{ page.sidebar}}
    </div>   
{% endif %}

Then in CSS use your parent/wrapper to style your blocks .my-sidebar .block { border: solid red; }

2) Individually create a block--block--[id].html.twig for each of your sidebar blocks and add my-desired-class here.