Self-assembly is a promising approach for achieving controlled nanoscale architectures in ceramics. The addition of ceramic-forming precursors to templating agents such as self-assembled surfactants or organic block copolymers (BCPs) has thus far been the primary route to forming ordered nanoporous oxides(1-5) and nanostructured non-oxide ceramics(6-9). In spite of its viability, however, this approach has several intrinsic shortcomings, including: (1) stringent requirements for amphiphilicity between template and precursor, lack of which may lead to macro-phase separation and loss of nano-scale order; (2) morphologies that can change uncontrollably with varying amounts of added ceramic precursor. Here we report a novel single-source ceramic precursor, based on a hybrid organic-inorganic BCP of polynorbornene-decaborane, that enables the formation of ordered ceramic nanostructures with tunable morphology and composition. In particular, we describe the synthesis of nanostructured boron carbonitride and mesoporous boron nitride, the latter of which exhibits the highest reported surface area for this material to date.