It is an interesting receiver to look over in the pictures. How ever the effect was achieved, maybe the heat was on the low side for quenching? There is little differentiating of colors between thick and thin areas, pin holes, etc.?
Pretty regularly, gray/black is passed of as case color background, so maybe the effect isn't so bad. I would guess the hope might have been to work more blue color in, and get that deep purple burgundy fade. In any event, there seems to have been some intention, and maybe a pro would have just redone it if it was not perceived as pleasing?