This is my mystery fish. Based on the feedback I've gotten so far, plus my ongoing attempts to master the vaguaries of angelfish genes and phenotypes, he (she?) appears to be a smokey zebra (leopard) marble veiltail, except that from what I understand, the smokey gene is supposed to keep the zebra markings from expressing on the body. And this fish clearly seems to have lines. Also, I don't really know which sort of marble he is. (The Norton marble articles keep scrambling my brain.)

For a while I thought maybe he just happened to have marbling that coincidentally formed lines, but then I noticed that some of the lines can fade when he's disturbed, and my understanding is that marbling doesn't fade. 

This photo was taken when I first got him. Since then he's developed more color; he now shows an area of blue sheen that starts behind the eye, spreads over most of the upper portion of his gill plate, and extends back past the gill plate onto the front part of his body.