But one could say prepare (say the chant of, but not release the spell) a fireball spell with arcane, then use black magic to create, say oil or some form of flammable gas near/around a target location, then release the fireball.
I'd figure as long as you don't actually mix the two magic types in the same spell it'd be fine, yeah?
Well, for starters it's not actually possible to prepare spells like that. One could speak part of the chant and/or weave part of the patterns associated with the spell, and then try to sustain this unfinished spell until finishing the incantation to invoke it, but even doing that much would increase the chance of the spell failing a lot, and would require the caster to concentrate quite a bit to remember exactly how the spell "feels" and continuously feed the spell magical energy (magical energy, when not bound to something relatively tangible, will always endeavor to disperse). "Finishing" a spell invokes it immediately, and performing the incantation without making it manifest will cause it to disperse on its own, no matter how much magical energy you pour into maintaining it, since you can't finish it anymore.
That said, if one were to attempt such a thing (one could try to do this; as mentioned above a spell not yet invoked can theoretically be sustained indefinitely), then one would still be "holding" a mass of energy infused into arcane nature, and another infused with black nature. While it technically would be possible to keep the two apart, I don't think there's a single mortal mind that would actually be capable of that kind of multitasking; the two energy-infusions would almost inevitably mingle, and the magic would backfire.
"If it's not specifically thought-control or the spell isn't sophisticated enough to separately null your knowledge of self and/or erase your memories of what you were feeling and sensing right before ... I'd expect any kind of generic-effect spell or abrupt illusion to be consciously very jarring and immediately identifiable, the latter especially when you're already aware that someone was casting a mind-affecting spell."
I never actually answered that question? Ugh... sorry. But it'd depend on the illusion-/mind-affecting spell, really. A lot of basic spells in that category would indeed be very jarring to the point where one would most likely realize what had happened - at least if one knew there was a possibility of such magic being invoked; otherwise one might just be really confused - but more advanced magic will usually make a more subtle transition. High-level illusions and mind-affecting magic is also either relatively gradual in its coming into effect, making the change less noticeable, or it will downright force it upon the target(s) that "you will not notice this". The former is obviously slower, but more reliable, whereas the latter is quicker but requires the spell to overpower the target(s) willpower in order for it to not be easily recognizable for what it is.
Incidentally, I'm also getting the feeling that our "translations" of arcane words are supposed to be very inaccurate, and their proper meanings do not really correspond to the English interpretations. (This has nothing to do with mind control.)
Oh yeah, the translations of the arcane language and Devil's Tongue are particularly inaccurate; their true meaning will depend greatly on the context they're put in.
