Graham Ekonime No The blocks are called in the included templates so {%use "my_blocks.twig" %} must be at the top of every template that uses those blocks
ie your file 404.twig
should be

{%use "/my_blocks.twig" %} {% block view_404 %} {% set view = "404" %} {{block('menu')}} {% set tail = '404' %} {{block('breadcrumbs')}} <div class="no_data"><b title="data not found">{{ translate ( '404_message' ) }}</b></div> {{block('breadcrumbs')}} {{block('search')}} {{block('online')}} {% endblock %}

because it calls 4 blocks (menu , breadcrumbs , search , online) that are defined in my_blocks.twig