Hey there, wanted to stop by and give my thoughts.
First off, I think there might be too much of a focus on Black being defined by acts of cruelty, sadism or willful malice. New Phyrexia I think does a good job about showing this with some of the abilities that the Praetors present, like the white praetor empowering some and degrading others, instead of empowering the whole group. Alot of what makes Black what it is, is simply about self-concern over others. Getting power is a part of that, and it can include using tactics like sacrificing and degrading others, but it doesn't mean said cruelty has to be enjoyed. Unless your someone like Rakdos (Black-Red), then it's about fun. There's even Sorin, whose been a bit of a hero at times in the MtG storyline and been black since the very beginning. Granted this is probably due to him being a vampire, but he does go White/Black in Innistrad, probably to represent him trying to restore order in his home plane.
I think adding Black to Alice would be about less of a concern about what her experiments have on others, but she doesn't quite have that desire for power that black usually goes for... the more I think about it, the less I'm having a hard time justifying Alice having that much black influence.
Yukari on the other hand, I think Black/White/Blue fits her really, really well. The Black/White is how she's maintaining order, but for her own gain over concern over others, and the Blue/White is more about how boundries require order, understanding and rules to create that separation that her power requires . Kinda ironic that a youkai would need structure, but I guess that's Yukari being that Youkai-like youkai for you. :/
Black also, while having death and weakening, doesn't mean that it's simply the realm of ghosts. Innistrad sets have spirits and ghost white colored, The Kamigawa block has
spirits of all colors. I think in this case, it might be better to look at characters like the Prismriver sisters more at who they are and what their powers do. Since Lunasa's music tends to bring down those who listen to her music to a calm attitude, but if too much can make them depressed, which would be black. Merlin's happy go lucky nature along with how her music tends to make people want to spontaneously dance is a bit red, and Lyrica's notes being illusionary along with her being a bit sly with her words makes her a bit more blue-ish. These were kinda hard to pull out, and Lunasa's is a reaaallllyy big stretch.
I could see Youmu being White/Green due to her guardian nature and being a gardener.
Chen maybe Red due to her childness and how he attacked the protags on a bit of an emotional impulse?
Faeries are kinda weird... they
used to be green a long time ago, but are mostly Blue now, with some black. I think that's because of the way the faeries control the board most the time. Plus, most the mischief stuff that faeries love in Touhou happens in red, not blue. I think Cirno would be blue due to how she can freeze stuff, which could be like "Tap X permanents. They do not untap until thier owner's next untap step". Dai could be blue since she's smart for a fairy or even red due to the fact she like to play tricks sometimes. Yup, very weird.
Also, Tenshi's definitely White/Red in my book. But I think that's because I'm reading her being more acting on emotion rather than self-centeredness. Black would work too since she's spoiled enough, but my guess is what's motivating her for the second color.
Here's what I'm using as a guiade, btw:
mtgsalvation.gamepedia.com/Color_Wheel. Found it to be a bit better of a guide for the color stuff since it condenses lots of article for this topic.
