How do I port my hook_alter module to D8? This is my D7 module (module name is ckeditor_show_blocks):

&lt;?php

/**
 * Implements hook_ckeditor_editor_settings_alter().
 */
function ckeditor_show_blocks_ckeditor_settings_alter(&amp;$settings) {
    $settings['startupOutlineBlocks'] = TRUE;
  }


All it does, it toggles Show Blocks by default on page load in the CKEditor when editing the page/node:



I'm trying to achieve same on D8. I did read some on hooks documentation, and it looked like same, so I didn't change anything, but the module won't apply when in editing mode, what I'm missing? Is hook format different? Does it have something to do with the fact that CKEditor is built-in feature in D8?

There is normal documentation on D7 on how to implement a simple hook module, but not so much about D8.
Considering that the Editor module in Drupal core is the equivalent of the Wysiwyg module for Drupal 7, and the core CkEditor module is the equivalent of the Drupal 7 module with the same name, I would say that the hook you are looking for is hook_editor_js_settings_alter() that, for a module whose machine name is ckeditor_show_blocks, it is implemented from the ckeditor_show_blocks_editor_js_settings_alter() function.

If you need to know the ID of the editor plugin used for a input format, you can get that from $settings['editor']['formats'][$format_id]['editor']; that is the case, for example, when you need to change settings if the used editor is the one implemented by the CkEditor module.

To make it clearer, the Editor module is the module that associate editors to the input formats used from a Drupal 8 site, while the CkEditor module in core contains an implementation of editor.