This question may not admit of precise answer. But obviously, a black person’s skin pigmentation offers substantial protection from the sun. What would the SPF equivalent be for the skin pigmentation of a medium-skinned black person such as, say, Will Smith? Of for a darker-skinned black such as Sidney Poitier?
Very black skin has a SPF of around 13.
There is a definitional problem and a practical problem with this.
I think the definition of SPF is the inverse of the transmission of a layer of the product applied as it’s supposed to be, for some weighted spectrum of ultraviolet tanning and burning wavelengths. So a product with a SPF of 10 means that the burning ability of the light reaching the skin is only 1/10 of what it would be without the product. You’re supposed to be able to use this by multiplying times by SPFs, so 20 minutes e exposure without any product is comparable to 200 minutes wearing the SPF=10 product. So, the definitional problem is that SPF should be measured by testing how much light makes it to the surface of the skin, and skin pigmentation only starts doing anything with the light AFTER the light gets to the surface.
The practical problem would be that the skin with the pigment in it is still being subjected to the tanning and burning wavelengths. It is only the skin underneath the pigment that gets protected by it.
Certainly, skin pigment provides protection from dangerous ultraviolet, but SPF seems like an obliquely useful measure of it at best. Perhaps a better comparison would be to measure skin damage in some way and then state the ratio of skin damage that occurs to two people with two different pigmentations.