How to access a previous matrix field value
As it turns out, you must use bracketed and not dotted syntax for matrix fields.

{% for contentBlock in entry.contentBlocks %}
    {% if contentBlock.prev %}
        {% if contentBlock.prev[matrixField] is defined %}
            {{ contentBlock.prev[matrixField] }}
        {% endif %}
    {% endif 
    ....
{% endfor %}


fixed this issue for me :)