Fluid (e.g. blood, urine, serous fluid in cysts) is an excellent transmitter of sound waves and appears black (= anechoic) on a sonogram. On greyscale images, prominent vasculature or vascular malformations can mimic hydronephrosis or cysts depending on the location. We should always obtain a few Doppler images when performing kidney ultrasound irrespective of the suspected etiology.
