In this paper, we are interested in a reconfigurable intelligent surface (RIS)-assisted symbiotic radio (SR) system, where an RIS assists a primary transmission by passive beamforming and simultaneously acts as an information transmitter by periodically adjusting its reflection coefficients. The above RIS functions innately enable a new multiplicative multiple access channel (M-MAC), where the primary and secondary signals are superposed in a multiplicative manner. To pursue the fundamental performance limits of M-MAC, we focus on the characterization of the capacity region for such a system when the direct link is blocked. Due to the reflection nature of RIS, the signal transmitted from the RIS should satisfy a passive reflection constraint. We consider two types of passive reflection constraints, one for the case that only the phases of the RIS can be adjusted, while the other for the case that both the amplitude and the phase can be adjusted. Under the passive reflection constraints at the RIS as well as the average power constraint at the primary transmitter (PTx), we characterize the capacity region of RIS-assisted SR. It is observed that: 1) the number of sum-rate-optimal points on the boundary of the capacity region is infinite; 2) for the rate pairs with the maximum sum rate, the optimal distribution of the amplitude of the primary signal is a continuous Rayleigh distribution, while for the remaining rate pairs on the capacity region boundary, the optimal amplitude of the primary signal is discrete; 3) the adjustment of the amplitude for the RIS can enlarge the capacity region as compared to the phase-adjusted-only case.