A novel Hadron Blind Detector (HBD) has been developed for an upgrade of the PHENIX experiment at RHIC. The HBD will allow a precise measurement of electron-positron pairs from the decay of the light vector mesons and the low and intermediate mass continuum in heavy ion collisions. The main HBD characteristics are: electron identification with very high efficiency (>90%), a moderate pion rejection factor (~100) and a material budget of <3% of a radiation length. The HBD is a windowless Cherenkov detector, operated with pure CF4 in a special proximity focus configuration. The detector consists of a 50 cm long radiator directly coupled to a triple Gas Electron Multiplier (GEM) detector with a CsI photocathode evaporated on the top face of the first GEM foil. Detailed studies of the detector performance will be presented. After the successful completion of a full-scale prototype, the final detector is presently under construction with installation for an engineering run expected in 2006.