Because Black Sex Links are hybrids produced by crossing various breeds of solid colored roosters with various breeds of barred hens (as well as hatchery quality Barred Rocks) they frequently have grey legs and can vary quite a bit in size. The pics below are some good examples and I could post...
With that copper feathering around the neck area, you have a Black Sex Links pullet. Black Jersey Giants are solid black. Black Sex Links are egg laying machines. You should get loads of large, brown eggs from that girl. :o)