What constitutes a Dark Art? What makes a potion or curse dark?

Question

It seems like not everything that can cause harm is considered part of the Dark Arts, so what does make a spell or a potion "DARK"?

For example, in Harry Potter and the Half-Blood Prince, the class brews the "Draught of Living Death" and this isn't considered dark, even though Slughorn says "one drop would kill us all".

Another example is the Reductor Curse; even though we never saw it in the books, I imagine it would cause quite a bit of damage to a human, yet it too isn't considered dark.

Answer 1

A bit speculative here, because I'm not sure there is a clear definitive answer available (happy to be proven wrong though).

Aside from the fact that elements of dark magic are typically suited for malevolent purposes, there seems to be some aspect of "finality" about them: the effects of dark magic usually can't be reversed or cured using most normal methods available to the average witch or wizard, and had effects that were not just harmful, but unstoppable or irreversible.

Some examples:

• The killing curse Avada Kedavra can't blocked or countered
• Fiendfyre can't be controlled or extinguished by normal means
• Wounds from attacks based on dark magic can't be healed even magically, but must be coped with: for example, George's ear cut by Sectumsempra, or the injuries that led to Moody's eye and leg replacements
• The curse that afflicted Dumbledore from the horcrux ring could only be temporarily contained, not cured or lifted

In contrast, a lot of the "light" curses or jinxes used by characters throughout the novels had fairly short-lasting impact, were easy to block or undo, and left no long-term effects.

Answer 2

I can't find anything specifically defining Dark Arts, but there is an old Pottermore article (original now gone, but archived) which gives some insight in reverse:

Spell Definitions
Every now and then somebody asks me for the difference between a spell, a charm and a hex. Within the Potter world, the boundaries are flexible, and I imagine that wizards may have their own ideas. Hermione-ish, however, I've always had a working theory:

Spell:
The generic term for a piece of magic.

Charm:
Does not fundamentally alter the properties of the subject of the spell, but adds, or changes, properties. Turning a teacup into a rat would be a spell, whereas making a teacup dance would be a charm. The grey area comes with things like 'Stunning Spells', which on balance I think are Charms, but which I call spells for alliterative effect.

Hexes:
Has a connotation of dark magic, as do jinxes, but of a minor sort. I see 'hex' as slightly worse. I usually use 'jinx' for spells whose effects are irritating but amusing.

Curses:
Reserved for the worst kinds of dark magic.

There are two points I think this gives us.

Firstly, the definitions are soft and your mileage may vary - different wizards would have their own ideas about whether a particular spell was a charm, a hex, or a curse.

Secondly, there is a sliding scale rather than absolutes - it is possible to have 'worst kind' of dark magic, so there can also be milder examples.

Putting these together, dark magic and the dark arts are not precisely defined. Some spells, such as the unforgivable curses, are legally defined in Britain as 'Dark Arts', but this is a decision, not an inherent property of the spell.

If the intent of the spell is to cause harm, it's shading more towards dark magic - the more harmful the spell is in intent, the more people who would call it dark magic.