The present invention relates to a device and a method for detecting blood or blood constituents in the liquid system of a device for extracorporeal blood treatment, comprising a dialysis device or filter divided by a semipermeable membrane into a first chamber and a second chamber, wherein the first chamber is part of the extracorporeal blood circulation system and the second chamber part of the liquid system of the extracorporeal blood treatment device. The device according to the present invention for detecting blood or blood constituents in the liquid system of an extracorporeal blood treatment device is designed as a unit for differentiating between the entry of blood into the liquid system due to a defect of the dialysis device or filter, for example a rupture of the semipermeable membrane of the dialysis device or filter, or the entry of hemoglobin into the liquid system due to hemolysis, wherein a differentiation is made between a defect of the dialysis device or filter or hemolysis based on the change in intensity of at least the blue fraction of the light exiting from the liquid.