I've always assumed as such: Roger was the true king during his days, and since Rayliegh was his right hand man he also deserved a similar epithet. And perhaps he may have had a somewhat royal presence (not literally but more in regards to his actions, his demeanor, how he went about things) on him. Additionally, I think because he was second to Roger, the one king, he was, to an extent, in his shadows - hence the dark part. However, as to why he wasn't called Dark Prince, it's mostly since there's simply just one true "Dark Prince" and Oda didn't to have Rayleigh's epithet be that since his fans would think Rayliegh is similar to Vegeta - when he's not. Thus Oda went with the 'Dark King' epithet.
Furthermore, as to him being the supreme Haki user, I highly doubt that since Roger shoul've been that. Or most likely Garp, sseeing as he is feared for his brute physical strenth.
Furthermore, as to him being the supreme Haki user, I highly doubt that since Roger shoul've been that. Or most likely Garp, sseeing as he is feared for his brute physical strenth.