It's not really a breed, it's a cross that produces black female chicks and a different color male chicks so they can be sorted without vent sexing at hatch. So it's impossible to say for certainty what you're getting other than black female chickens.
Most hens are not human aggressive at all. It's roosters that get a little too territorial that become aggressive. If you want to guarantee only hens, a sexlink is the best way to go. Black sexlink girls are all black at hatch, some may have a bit of white on their necks. A red sexlink girl will be gold or red.
They are usually a Rhode Island Red and Barred Rock cross. Each hatchery has it's own particular lines and combinations of breeds. Hatchery birds are bred mostly for a high production rate. Temperament is rarely considered, so understand that they might be outgoing and want to be around you all the time, or they could be flighty and skittish.
Black Sex Links are produced by crossing red gene roosters with barred gene hens (virtually all hatcheries use either RIR or Production Red roosters and Barred Rock hens). Not only can the chicks be sexed by color at hatching (male chicks have a white spot on top of their heads, female chicks have solid black heads), but they are egg laying machines, outlaying either parent breed. It's one of the interesting quirks of hybridization. Black Sex Links are sometimes marketed by hatcheries under the labels Black Star, Bovans Nera, or Rock Red). I've raised Black Sex Links for years (my favorite chickens) and they have always been very friendly and done quite well with my children and granddaughter (pictured in my avatar). In addition, they are very hardy and have been my best layers (out of dozens of breeds and hybrids), consistently churning out more than 300 eggs per hen per year. I don't think you can go wrong with Black Sex Links. )