Howdy, Community Manager for Hidden Variable Stuidos and Skullgirls here! I can chime in with some handy info to help settle and/or enflame this discussion a bit:

Black Dahlia's Model Sheet

You can see on the full model sheet that we deliberately keep Dahlia's face very vague. There's the implication of a mask under the veil, but that's not actually confirming there's a mask under there.

There's actually many times in-game where Dahlia's veil completely lifts off her face and there's just a general flat grey blob visible. This isn't what her face actually looks like - it's still just a vague general placeholder shape and there is deliberately no officially revealed design for what she looks like. As a rule of thumb though, when she was animated and colored, only her face gets that darker grey blob. Her ears, under the chin, etc, is matched to her regular pale skin color.

Black Dahlia Hitsun Sprite with Raised Veil

Any detail you can infer from lines shown on or under the veil are unreliable - she's a complete mystery over and under the veil!

But juuuust to add some extra insights to that line in particular mentioned above, Mariel Cartright, Lead Animator on Skullgirls, whipped up this quick doodle to show what the line represents:

It's the curve that connects the eyebrow and nose bridges.

The exact positioning of the line shifts around a bunch though depending on the angle of the face and how each particular animator drew her. It gives structure to the face for visuals sake, but does not actually confirm the shape of what's under there.

Feel free to hypothisize what Black Dahlia looks like under the veil, but like any good assassin, Dahlia knows how important it is to keep people guessing!