In situ x-ray diffraction and wave-profile measurements were carried out on polycrystalline boron carbide under laser- induced shock compression at 51 GPa at the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. The diffraction data indicate that boron carbide remains crystalline to this pressure and there is no evidence for a major structural phase transition. The peak elastic stress for boron carbide was found to be 15.9 GPa, in agreement with previous gas-gun measurements, despite differences in sample thickness and loading rate between laser and gas-gun compression. The starting sample contained excess carbon in the form of graphite. The graphite peaks disappeared upon compression of the sample, indicating that carbon was incorporated into the structure of boron carbide behind the shock front and retained upon decompression.