Multiferroic CuO showing two distinct antiferromagnetic (AFM) phases: incommensurate (ICM) spiral and commensurate (CM) collinear AFM phases, is investigated by resonant x-ray diffraction (RXD) at the Cu L₃ edge. In the ICM phase where spin-spiral-induced ferroelectricity develops, circular dichroism ascribed to the handedness of the spiral magnetic order is observed at the magnetic Bragg (0.506, 0, -0.483) reflection. By measuring two-dimensional maps of the reflection intensity, the spatial distribution of the ferroelectric domains is clarified and is found to be strongly influenced by crystallographic twin structures. In the CM phase, weaker but substantial circular dichroic signals are observed at the magnetic Bragg (0.5, 0, -0.5) reflection. It is found that the spatial distributions of the circular dichroic signals resemble those in the ICM phase, suggesting some relation between the ICM and CM phases. The origin of the circular dichroic RXD observed in CuO is discussed based on the experimental observations.