Black perception (perceived blackness of gray 0) of transparent OLED displays was studied in this paper. In pre-test, maximum luminances of acceptable black level under various surround conditions were found in a non-transparent display. In the first experiment, the luminance of a transparent patch was compared with that of an opaque one in order to find the effect of transparency on black perception. As a result, participants perceived the transparent patch darker than the opaque one even when the two were in similar luminance levels, which we termed as the "Transparency Effect." In the second experiment, the perceived brightness of gray 0 with various background brightness conditions was investigated to observe the effect of induced black perception. Most participants perceived the luminance of gray 0 darker with brighter background luminance, but some did not. It might result from transparency of gray 0 which had a role as a window presenting the area overlapped with a transparent OLED display.