In a cognitive radio network, a primary user (PU) shares its spectrum with secondary users (SUs) temporally and spatially, while allowing for some interference. We consider the problem of estimating the interference coverage region of the PU, i.e., the region outside of which SUs may utilize the PU's spectrum regardless of whether the PU is transmitting or not. We propose a distributed boundary estimation algorithm that allows SUs to estimate the boundary of the coverage region collaboratively through message passing between SUs. We also propose a spatial spectrum sensing scheme based on the estimated boundary. Simulation results suggest that our proposed scheme has better estimation performance and communication cost trade-offs compared to centralized boundary estimation methods, and has better weighted throughputs than traditional fusion center based collaborative spectrum sensing methods.