i cant give you a web site but have you ever thought that maybe you have some relatives that were mongolian?

I adopted my son from Guatemala. He had a spot about the size of a quarter on his upper arm. He's 4 now and it has almost disappeared on it's own. We were told when we adopted him that is was called a "Mongolian" spot and that it was very common in Central America. The fact that it was on his arm was the only oddity since we were told that the spot is usually on the back or buttocks. You might want to look up "Mongolian Spot" even though you don't think you have any Mongolian heritage. I think that is basically just the name of the spot and not necessarily "proof positive" that a person is Mongolian.

God Bless

Mongolian spots are nothing more than dense collections of melanocytes, the skin cells which contain melanin, the normal pigment of the skin. When the melanocytes are close to the surface, they look deep brown. The deeper they are in the skin, the more bluish they look. Either way, they are not related to bruises or any other medical condition. They do not predispose to skin cancer or any other problem.

Who gets it?
At least one Mongolian spot is present on the great majority of babies of Native American, African, Asian, or Hispanic descent. They are also present in about one in ten fair-skinned infants.

Despite the name, Mongolian spots have no known anthropologic significance, except for being more common in darker-skinned infants.

These flat birthmarks can be deep brown, slate gray, or blue-black in color. They do sometimes look like bruises. The edges are often, but not always, indistinct. They are most common on the lower back and buttocks, but are often found on the legs, back, sides, and shoulders. They vary from the size of a pinhead to six inches or more across. A child may have one or several.
Mongolian spots are present at birth, and most of them fade (at least somewhat) by age two. Most have completely disappeared by age five. If Mongolian spots remain at puberty, they are likely to be permanent. Fewer than five percent of children with Mongolian spots still have any by the time they reach adulthood. Those who do tend to be the ones with multiple, widespread spots, or with spots in unusual locations.
They are not contagious, so no treatment or prevention is necessary. They can sometimes be misdiagnosed as bruises on babies, and often the parents are accused of child abuse, when they did nothing wrong.

Mongolian Spots: a type of birthmark, colored skin spots that either are present at birth or develop shortly after birth. Birthmarks can be many different colors, including brown, tan, black, pale blue, pink, white, red, or purple. The cause of most birthmarks is unknown. Most birthmarks are not inherited.
A "pigmented"–or colored–birthmark, Mongolian spots usually are bluish and appear as bruises. They often appear on the buttocks and/or lower back, but they sometimes also appear on the trunk or arms. The spots are seen most often in people who have darker skin.
The cause of pigmented birthmarks is not known. However, the amount and location of melanin (a substance that determines skin color) determines the color of pigmented birthmarks. Symptoms of pigmented birthmarks include skin that is abnormally dark or light, or bluish, brown, black or blue-gray in color. Discolorations of the skin may vary in size and can be smooth, flat, raised or wrinkled. Pigmented birthmarks may increase in size, change colors, become itchy, and may occasionally bleed.
In most cases, no treatment is needed for the birthmarks themselves. When birthmarks do require treatment, however, that treatment varies based on the kind of birthmark and its related conditions.
Since there is an increased risk of skin cancer in congenital nevi (moles that are present at birth), see a doctor if you notice a change in color, size, or texture of a mole or other skin lesion. Also, see a doctor right away if there is any pain, bleeding, itching, inflammation, or ulceration of a congenital mole or other skin lesion. There is no known way to prevent birthmarks. People with birthmarks should use a good quality sunscreen with a high SPF when outdoors in order to prevent complications.

Mayans also get a mongolian spot. So if you are from Guatemala, then you may be at least part mayan.

I have a Maya friend whose baby had a classic "Mongolian spot". The spot was at the base of the spine, and was not black, but the color of grape juice. It gradually fades away in a couple of years. This is common.

A black spot is not common.