I suspect it's nothing so exotic. Iirc the spell invisibility doesn't work by bending light, or super transparency, or mental illusions. It makes you invisible by making you invisible. This also applies to Windsight, because you are now invisible.
It's probable this is just a similar effect. Make you more attractive, or perceived as such. This includes to the sense of Windsight, and this is just the best way to make Dhar seem attractive to Mathilde.
An interesting thought actually, is this because Mathilde finds magical power under Windsight attractive, so it's playing off that? If someone didn't feel that, and also wasn't disgusted by the Dhar, would they even have that sense warped at all?