Spectrum Sharing Data Falsification (SSDF) attacks can cause heavy performance degradation to Cognitive Radio (CR) based Internet of Battlefield Things (IoBT) networks. The challenge in such networks is to handle this security problem real time, in addition to spectrum sharing. This requires a robust CR architecture and protocol that can provide integrity of the spectrum sensing data being shared between Secondary Users (SUs) for collaborative spectrum decisions in protocols like PROLEMus. We propose one such protocol called Proactive Blockchain based Spectrum Sharing (ProBLeSS) protocol which leverages a blockchain to provide security against SSDF attacks in CR-IoBT networks.