Just from observation plus genetics, it must be black, as it is also a dominant gene and the sole hair colour of two races at least. In fact, because blonde hair is recessive it's predicted to gradually die out altogether in the near future.
In human examples, the simplest example for hair color involves several alleles; but the most common hair color in humans world-wide, black, is produced by the double-recessive. Common does not necessarily mean dominant! http://plantphys.info/principles/seedshape.html