Nanostructured polycrystalline cubic boron nitride, an advanced superhard and refractory covalent solid with hardness approaching that of diamond, exhibits extreme mechanical properties due to high chemical and phase purity, ideal crystallinity and high intergranular adhesion. The non-trivial Raman fingerprint of nano-cBN shows both bulk and surface/grain boundary modes in the phonon density of states. The anomalous heat capacity of nano-cBN is comparable to that of nanostructured metal bulks and shows universal behavior with a maximum at 20-25 percent of the Debye temperature. The intergrain boundaries and their intrinsic stacking-fault structure are a reliable, but probably not unique cause of excessive heat capacity. The excess heat capacity is typically interpreted as arising from disorder-induced softening of vibrational modes in grain-boundary regions with reduced bond stiffness, without explicit identification of disorder nature. Our results indicate that nanostructured cubic boron nitride hosts grain-boundary phonon modes that give rise to an anomalous excess heat capacity, a universal effect previously known only in metals with similar space-filling polycrystal organization.