In keeping in line with holistic, natural, organic product-making, I'd say neither petroleum or mineral oil is acceptable or appropriate for use in alleviating ashy/dry/thirsty melanin-rich skin. In fact, there is extensive research that shows that both of these ingredients are harmful and toxic to the body.
I agree, though, that Shea butter and Cocoa butter are wonderful staples to keep in a brown skinned persons "medicine cabinet;" however, I believe using these alone or even combined will not combat the dryness that melanin-rich skin can endure.
I recommend red Palm oil or Palm kernels, sustainably sourced, of course. Additionally, babassu oil, kokum butter and mango butter *in their natural states and as unrefined as possible* are excellent emollients for melanin-rich skin. Consider adding essential oils like myrrh and frankincense, which aid in providing anti-aging elements to the final product. Win-win!