In all honesty, the best sunscreen for dark skin to prevent tanning is exactly the same as those with light skin. All sun creams work the same regardless of the skin color they are being applied to. Quite often those with darker skin do not burn as easily and are not impacted as easily by the sun, but if you want to prevent dark skin from tanning, you should follow the exact same steps as anyone with lighter skin and ensure you are using an SPF 50 cream and applying this regularly.