Foxface Lo is an outdated name. The genus Lo has been discontinued and now isconsolodated into Siganus. The difference between "foxface" and "one spot foxface" is argued by taxonomists. Two opinions. 1) they look different so they are different species 2) they are genetically almost indistinguishable so they are the same species. Jury is still out.
That being said, there is no "blackbar" in the genus. Like many reef fish, the foxface is capable of changing color and patterning. At night, they take on an almost pure black and white mottling characteristic of US military camouflage (before the change to digital camo). When stressed, the yellow turns darker, sometimes solid black. That being said, with every fish havin it's own individual personality, it's very possible that her foxface is not stressed, but simply likes to be half black. Just to be different.
But it's not a different species or even color morph. It's either a one spot or a "normal" foxface rabbitfish.
Ps. Glancing over at my tank, my one spot is darker on top right now. Not solid black. One side is very dark yellow, the other side is dark and normal yellow striped. But only on his top half. He's also telling me it's feeding time.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef