An optical blood monitoring system for blocking unwanted light from reaching sensors in a sensor clip assembly fastened to a blood chamber connected in an extracorporeal blood treatment system. The sensor clip assembly includes opposing heads with LED emitters and photodetectors. In one embodiment, lenses in the heads are surrounded by shrouds extending from the lenses so that when the sensor clip assembly is fastened to the blood chamber the shrouds block unwanted light from reaching the photodetectors. Either alternatively or as a complement to the shrouds, the blood chamber includes an opaque portion or a portion colored to attenuate particular wavelengths of light to further enhance the overall ability of the blood chamber and sensor clip assembly to block unwanted light from reaching the photodetectors.