When I'm looking to the fade targets and the effects over the last months I always observe the same - that the black inks at the end fade into some darker or lighter brown, except on the HP premium photo paper, and the black ink fades faster than cyan or magenta. There is probably a reason for it , the black color has to catch most of the photons with all the energy levels/wavelengths, the other colors only a part of it. The Fujifilm inks have shown the best overall dye ink performance, the Fujifilm black has a better black level than the Epson Ultrachrome D6 black , that's the status after print but not after months of exposure. When the Cyans and Magentas fade in an image the print may keep the color balance somewhat, and only getting lighter overall, but then the black turns into brown, changes this color balance and shifts it to a sunburn look. I'm asking myself what a quasi-black, mixed from cyan and magenta, would do under these conditions. The blacklevel would not be as dark as with a genuine black ink to start with, o.k. but what after a longer exposure period - could it be that the overall look is better since that the ugly brown shift of the black ink does not show up ?.
The lower end color printers with 4 colors, with a pigment black for normal paper, all models by Canon or Brother or Epson, mix the inks to black when selecting and printing on photo paper , this in the first place to offset the missing photo black, and probably not for long term light exposure.
I think there is something left for a test, a longer term test to get some impression how such mixed black would perform.