Narrowband Internet of Things (NB-IoT) has been found to be a prime facilitator of massive machine-like communications in smart cities, in health care, in logistics, and in industrial automation. The use of it is however marred by grave security and privacy issues mostly due to its mass scale use, low power as well as its dependence on special centralised authentication systems. The conventional security mechanisms, including TLS or DTLS, have high computational complexity, higher power usage, and prone to latency, and therefore cannot be applied to NB-IoT devices that use few resources. Speaking of which, existing blockchain-based security solutions, even though decentralised, consume resources, which makes them unsuitable to limited NB-IoT networks and introduces performance and scalability bottlenecks. These deficiencies result in poor performance including spoofing, eavesdropping, and unauthorised access, thereby deteriorating the performance of NB-IoT communication when used in real-time services. To resolve them, we introduce a new protocol stack, BLPSec-NB, a Blockchain-Enabled Lightweight Protocol Stack, to offer security to the inter-device communication in the NB-IoT networks. The suggested solution is based on the two-layer blockchain-based solution in which a local (micro)chain with a Directed Acyclic Graph (DAG)-based local device authentication in an area is used. It also uses an expandable regional blockchain snapshot to administer trust at the region. A new consensus mechanism, named Trust Score Consensus Mechanism (TSCM), will be used instead of a less efficient consensus algorithm by means of a more advanced key-generation scheme at the physical layer (PLKG). Besides, adaptive cross-layer security model has the ability to dynamically change encryption complexity depending on the level of energy that the device has and sensitivity of the data that it has to balance the data needs and device energy. The effectiveness and feasibility of BLPSec-NB are proved by simulation results. Unlike the TLS and IOTA security protocols, it will lead to a 65% decrease in the energy usage, a 90% decrease in the latency of transactions, and a total 88% enhancement of packet integrity detection. The approach offers secure, energy conscious communication which is scalable and suitable with the use of real time NB-IoT applications. BLPSec-NB is a futuristic lightweight blockchain framework of NB-IoT that is resistant to cyberattacks and low power consumption with high scalability to support future IoT ecosystems.