SPF is an abbreviation for Sun Protection Factor
SPF does differ for different skin colors, not as far as sunscreen goes, but as far as a person's own skin goes. Darker-skinned people have more melanin in their skin than lighter-skinned people. These varying levels are what makes different skin tones and shades. If you've got darker skin, you have a higher natural tolerance to withstand sun exposure than people who are paler than you in skin tone.
Here's
an article from the Washington Post that talks about the FDA, sunscreen and the chemicals in it.
As far as natural sunscreens go, do their ingredients protect you against UVA and UVB radiation? If they do, great, they'll work. If not, you'll probably just wind up aplying something oily to yourself and burning more.