This peafowl color came up in a facebook discussion today. I had heard a bit of the story before, but hadn't seen pics, or if they were given a name. Today I saw pics and the name, and searched here to see if they were ever mentioned.
This color is derived from hybridization with Congo peafowl that occurred decades ago. Females in the first generation were sterile, but males were fertile, and bred back to regular IB. Over a couple of generations, female fertility was restored, and the line was carefully bred to both preserve the increase in color and maintain fertility in hens. BTW, the hens have fully blue necks, and one could mistake them for males. I saw some pics of such hens -- stunning. But I don't think this is inherited as a simple mutation -- rather, it is a strain. This means that crossing to non-Ultramarine won't give "splits", but would rather be like crossing breeds.