My Daughter Has Unexplained Dark Blemishes Apearred On Her Knees,

My daughter has unexplained dark blemishes apearred on her knees, they look like bruises but she hasn't knocked or hurt herself. Her legs hurt and appear a bit swollen.I took her to out of hours surgery they did xray and blood tests they couldnt find anything, but the blemishes are spreading down her leg. Can you advise?