Dark spots are the result of excess melanin production in a concentrated area. Some birthmarks are caused by high skin pigmentation. And other forms of dark spots (e.g. age/sun spots) occur when the skin produces higher levels of melanin in response to sun exposure. This is how it tries to protect itself since melanin is like a type of sunscreen that your body manufactures.
Fraxel Dual uses the 1550nm/1927nm wavelengths to break up this pigment in specific areas without being absorbed by the surrounding melanin in other areas. This makes it safe for all skin tones. Fraxel lasers can be used on just about any part of the body. Nd:YAG lasers especially the Q switch modes are also effective in targeting dark pigments specifically. The laser is absorbed by the pigments which are then destroyed. Your specialist laser physician should be able to determine the best treatment approach for your dark patch