That doesn't really make much sense, when you think about it; the notion of youkai as a whole being able to smell blood thing. It really seems like we're going back to the whole "youkai are a species" mentality. It'd be astounding if, among the millions of youkai species, each and everyone of them could smell blood and, even more astoundingly, could distinguish between human blood and only human blood from any other ambiguous source. It just seems rather strange since around 50-90% of youkai are unrelated as a species and, as such, each and every one of them sharing such a trait while few animals or humans, from which various youkai are derived or created, cannot. Plus, in terms of adaptation, there's very little practical reason for most youkai to be able to have such a honed sense of smell, especially where blood is concerned.
But Witches and Wizards/Mages aren't always human. According to the R&V wiki, which I am assuming is a basis dependable basis for all of this, they contain similarities, and thus it is speculated that they may be a human subspecies, but nothing is verified as of yet.
In terms of real life lore, it varies between locales, but for the most part (at least, in terms of popular culture) witches are split up into those born with magic capabilities (those who are not human), and those who are human and are capable of magic (in other words, the magic is accessible to all). Now, there can be half-breeds, as I assume humans can fertilize witches (due to the supposed similarities), in terms of the former.
Still, I don't think that witches or wizards/mages, at least in terms of Youkai Academy, are human in any way, unless they are mixed by some means (human father and magic mother, or vice versa). This is pretty much speculation, since it's Sapphire's RP, but I'm just saiyan.
...Man, I've been watching too much Dragon Ball -.-
Oh, and my post will be up tomorrow, I don't have the drive to write it tonight, sorry for the delay.