Believed to be some sort of Olive Egger..really?
Technically sex link is not a breed - they are a mixed breed bird that has been created by crossing a barred hen with a non-barred rooster (most common in hatchery birds is a red based rooster). The resulting chicks hatch with females being all black and males being black with a white/yellow spot on the head. The head spot is an indicator of barring - the male chicks inherit the barring from the mother bird, the female chicks do not. The females feather out black with some degree of leakage like the colorful feathering on your girl - some have very minimal leakage and others get a lovely pattern over a large part of their body.