How to use twig parent function on nested blocks
Try to keep the blocks separate in your child template. That's how it's done in a comparable example in the Twig Documentation.

Parent Template:

{% block breadcrumbs %}
    {% block crumb %} 
    {% endblock %}
{% endblock %}


Child Template:

{% extends "parent.html" %}

{% block breadcrumbs %}
    {{ parent() }}
{% endblock %}

{% block crumb %}
    My custom content
{% endblock %}


.

Edit:

If you're not adding any content to your "breadcrumbs" block from the child template, you can also omit that block completely:

{% extends "parent.html" %}

{% block crumb %}
    My custom content
{% endblock %}