Twig 101
You can nest blocks inside each other:
// my-template.twig
{% block test_block %}
{% if myCondition %}
{% block test_markup %}
<a href="#">link</a>
{% endblock %}
{% endif %}
{% endblock %}
This way if you have a different condition in another template you can still reuse the outer block without repeating all the content:
// my-other-template.twig
{% block test_block %}
{% if myNewCondition %}
{{ block('test_markup', 'my-template.twig') }}
{% endif %}
{% endblock %}