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There were no after-effects of magic on Mary, since Mulciber wasn't successful at what he did. Obviously Mary would have been upset, but that doesn't prove it was dark magic specifically. Nor does it mean that Lily's interpretation of what happened that she herself did not see - which would have been as you say colored by Mary's distress - was any more correct in the facts than Severus' was.
And no, the text does not clearly state what Dark magic is. Where are the quotes as to what makes something dark or not, please? What differentiates dark magic from other magic beyond an individual character saying they think a certain spell was dark? (Never with any explanation as to why.) We only know what individual characters and some institutions consider to be dark, and not all of those opinions are congruent. Dark magic isn't just hurtful magic - or scourgify would qualify because it can be used to waterboard someone. Sectumsempra could be used to cut up plants, OTOH - does that mean it isn't dark or is that irrelevant according to the theory of dark magic? Are the twins' Ton-Tongue Toffees dark magic? Nothing to say either way according to any theory of magic.
The Unforgivables are dark everyone agrees, but we are never told by what formal definition of dark magic they qualify as such - whether it is the harm they do to the victim, to the caster, what is required of the caster to cast them, nothing specific is pointed out as a reason they are dark magic specifically. And their being dark doesn't give a definition of the dark arts as a whole. Grapes are fruits but they don't define the category 'fruit' by themselves. Your *interpretation* of dark magic works for a certain reading of the text, but it is an *interpretation.* The text itself never gives a clear, coherent definition of what makes one spell dark and another not-dark. It only gives some characters' occasionally contradictory opinions on the subject.
And no, the text does not clearly state what Dark magic is. Where are the quotes as to what makes something dark or not, please? What differentiates dark magic from other magic beyond an individual character saying they think a certain spell was dark? (Never with any explanation as to why.) We only know what individual characters and some institutions consider to be dark, and not all of those opinions are congruent. Dark magic isn't just hurtful magic - or scourgify would qualify because it can be used to waterboard someone. Sectumsempra could be used to cut up plants, OTOH - does that mean it isn't dark or is that irrelevant according to the theory of dark magic? Are the twins' Ton-Tongue Toffees dark magic? Nothing to say either way according to any theory of magic.
The Unforgivables are dark everyone agrees, but we are never told by what formal definition of dark magic they qualify as such - whether it is the harm they do to the victim, to the caster, what is required of the caster to cast them, nothing specific is pointed out as a reason they are dark magic specifically. And their being dark doesn't give a definition of the dark arts as a whole. Grapes are fruits but they don't define the category 'fruit' by themselves. Your *interpretation* of dark magic works for a certain reading of the text, but it is an *interpretation.* The text itself never gives a clear, coherent definition of what makes one spell dark and another not-dark. It only gives some characters' occasionally contradictory opinions on the subject.