The problem with this kind of skill system is that the magical categories are very, very broad. It wouldn't be quite as bad as assuming that all STEM subjects are the same, but "Charms" is merely a catch-all for several branches of magic, some of whom have overlap, granted, but that's akin to saying that Physics, Engineering, and Chemistry all use Mathematics, so it's the same.
Some of the categories (mind, this is all head-canon, but I like to think that it's the constructive kind):
- Mind-affecting Magic: At the low level, there's Cheering Charms, while higher levels have Memory Charms, or even Occlumency and Legilimency.
- Illusions: The Disillusionment Charm is likely the most well-known, but we know that there's spells to make things looks like something else, like walls pretending they're doors from Hogwarts itself.
- Elemental Spells: Aguamenti should technically be Conjuration, but it is classified as a Charm, and we've seen spells related to fire (especially fire), ice, and lightning, so I put them into one category in my mind, though it's more of a "the thing they've got in common is that they don't really fit well other places", in that I believe that being truly good at a branch of elemental spells require some affinity with it. Dumbledore's pretty good with fire, for example, which fits well with his having a Phoenix as a familiar.
- Divination Spells: No, not telling the future, but rather getting information. The Four-Point Spell would be here, as would age lines.
- Environmental-affecting Spells: This one is fairly obvious. Making space larger or smaller, making an entrance magically locked, shrinking or engorging things. In short, adding properties to an area.
- Object-affecting Spells: Lots of overlap with the category above, but I chose to make the distinction. Summoning and banishing are affecting the environment, but you can hardly summon a whole room; you need at least one target for it to work properly.
There can easily be overlap between them, of course. But tell me, how many of these categories could have spells in them that's considered dark? Legilimency would likely be heavily restricted, if not outright forbidden, a person with a strong affinity for fire spells would likely have greater control over Fiendfyre, and most Curses could be said to be affecting an object, or even an environment.
Having a Dark Arts skill is... I don't really know. Dark Arts isn't just a bunch of incantations that just require an incantation and some negative emotion to achieve results to achieve results much quicker. I subscribe to the thought that negative emotions is what separates "true Dark Magic" (as opposed to magic restricted by the ministry), in that it has a cost to your soul/sanity to use. We know that murder tears the soul, but that powerful magic can be performed by making use of this tear, likely to a rather permanent detriment, so I don't think that it's a stretch to say that lower-level Dark Magic exists that make use of the same.
I'm rambling a bit, but my point is that Dark Arts can't really be adequately represented by one skill. Being good at, say, a bunch of nasty curses wouldn't translate to proficiency with the Imperius skill, for example.
Ah well, Ashton is still cool. Watched!
