Any idea what breed this is?

[Emrosenagel]

She does not look like a Serama at all. I would ask her what other breeds she has. Can we see picture of your other two girls?

 

[MysteryChicken]

But it is normal for whatever breed she might be, right? It doesn’t mean she’s sick or something?

It's just color pigmentation of the skin from extension of Black. It can appear on any bird with black a base: Birchen, Black, Blue, Splash, & Lavender are some it can appear on.