Your question about why only dark tanned skins look realistic has another aspect to it.

Most skin textures that come with characters are smooth - human skin isn't, in 2 different ways.

Firstly, the pores on human skin add a roughness to it
Secondly, if you look at skin under a bright light, different areas of skin absorb/reflect the light at slightly different rates.

Dark tanned skin conceal both of the above, which is one of the reasons why tanned Poser skin looks more authentic.