How can I return only 1 random block in a Matrix in Craft using Twig?
You can create a random order by including rand() in your OrderBy function.

{% set randomBlock = entry.carouselMatrix.orderBy('rand()').one() %}
{% if randomBlock is not null %}
    {% set randomSlide = randomBlock.slides.orderBy('rand()').one() %}
    {% if randomSlide is not null %}
        {{ randomSlide .caption }}
        {% set image = randomSlide.image.one() %}
        {{ image is not null? image.getUrl() : '' }}
    {% endif %}
{% endif %}