Bright and dark angels were originally separate, distantly related races.  I had at first classified them as a pair of magical races, dependant on their unique magical affinities to live.  The more I delved into the magic system, the more that began to make absolutely no sense.  As they are now, they are still dependant on their affinities to live – and when the one gives out, they go from bright to dark, and die shortly thereafter – but as their affinities are also the cause of the affliction that warps their bodies, they can be born to any race.