Fetch fields of a block defined via content type programmatically
You are in the wrong entity. block is a config entity to configure where and how the block is displayed and which plugin is used to build the block.

The plugin can be a core, contrib or custom block class. In your case a core plugin, which fetches content from the database. The content is stored in the content entity block_content. 

So the entity type to modify the fields of a custom block is block_content and you have to use this hook:

hook_block_content_view_alter(&amp;$build, $block_content)