Apparently that's not unheard of among the Mongols; they tend to show a greater range of physical attributes, particularly in terms of hair and eye colour, than other East Asians. I've been told that it may be to do with their nomadic history- such peoples tend to breed more regularly with those outside of their native ethnic group, particularly with other nomadic peoples, thus providing a more diverse genetic background. The average Briton's ancestors may be British back into the neolithic, but the average Mongol may well have ancestors ranging from Korea to Ukraine.
Of course, it may also be entirely indigenous; there are other typically black-haired peoples who throw up a red-head now and then, such as certain Pacific Islanders and, as Lord Baal mentioned, some South American peoples.