Entries related to BOTH categories
You can limit to only products that have multiple related categories like so:



{% set dressesCat = craft.categories...first() %}
{% set blackCat = craft.categories...first() %}

{% set products = craft.commerce.products({
    relatedTo: {
        targetElement: ['and', dressesCat, blackCat],
        field: 'productCategories'
    }
} %}