We present the results of prompt optical follow-up of the electromagnetic counterpart of the gravitational-wave event GW170817 by the Transient Optical Robotic Observatory of the South Collaboration. We detected highly significant dimming in the light curves of the counterpart ({Delta}g=0.17+/-0.03mag, {Delta}r=0.14+/-0.02mag, {Delta}i=0.10+/-0.03mag) over the course of only 80 minutes of observations obtained ~35hr after the trigger with the T80-South telescope. A second epoch of observations, obtained ~59hr after the event with the EABA 1.5m telescope, confirms the fast fading nature of the transient. The observed colors of the counterpart suggest that this event was a "blue kilonova" relatively free of lanthanides.