Problem/Motivation
Fixing #1899772: Fix name retrieving for block plugins to fix hook_block_view_NAME_alter() I've realised that ID and NAME on hook_block_view_ID_alter and hook_block_view_NAME_alter are quite confusing to work with.
According to this code (with the fix above included)
// All blocks, even when empty, should be available for altering.
$id = str_replace(':', '__', $entity->get('plugin'));
list(, $name) = explode('.', $entity->id());
drupal_alter(array('block_view', "block_view_$id", "block_view_$name"), $build[$entity_id], $entity);$id is actually referring to the plugin, which can be shared by many instances and the $name is actually the id, making the twist extra confusing.
Proposed resolution
Replacing $id and hook_block_view_ID_alter by $plugin and hook_block_view_PLUGIN_alter.
Replacing $name and hook_block_view_NAME_alter by $id and hook_block_view_ID_alter.
Remaining tasks
Once #1899772: Fix name retrieving for block plugins to fix hook_block_view_NAME_alter() gets in, apply the suggested changes in BlockRenderController.php, BlockTest.php, block.api.php and block_test_alter hidden submodule.