It would be pretty interesting to here a member of House Tindomiel's philosophical position on Dark Magic, if indeed they do consider it acceptable to use it in some circumstances*. Mathilde considered Frederick van Hal a tragic hero in some ways, so she may be more open to understanding, if not agreeing with whatever their position is.
* elves' apparently resistance to mutation and their philosophy of using the Winds might (or might not) allow them to safely use it in limited controlled circumstances, like we can use radioactive materials. Considering non-dark elves' consider it a sad necessity to sometimes embody Khaine and kill someone, perhaps they also sometimes consider it a sad necessity to embody Hekarti and use naturally occurring dark magic (perhaps at one remove, using Hakarti's favour and the blood sacrifices involved), to do something important enough.
Wild speculation, but I wonder if dark elf sorcery is something much more like Necromancy than is usually understood, using tongs of Hekarti's divine power to manipulate Dhar at one remove. As divine power isn't one of the Winds, the tongs wouldn't be corrupted. Something one of the descriptions of dark elf sorcery pretty explicitly specifies is that they don't consider using the individual Winds sorcery**, just their use of Dark Magic, Wind casting is something else.
** which in turn opens up the potential for male dark elf spellcasters that don't use Dhar, as it's specifically male sorcerers that are forbidden.
We have seen magic-attracting runes, didn't we?
I think we saw magic repelling rather than magic attracting runes.