Blockchain is an emerging technology that uses distributed ledgers for transparent, reliable, and traceable information exchange among network nodes. Blockchain and its 3rd generation Tangle-based implementations quickly extend their territory beyond crypto-currency to a broad range of applications using fee-less transactions over the Internet of things (IoT). However, this technology suffers from sluggishness in consensus-based validation of information that restricts its applicability to time-sensitive applications such as smart health. In this letter, we propose an optimized policy for sampling rate by IoT sensors that utilize blockchain and Tangle technologies for their transmission with the goal of minimizing the age of information (AoI) experienced by the end-users, considering both processing and networking resource constraints. Simulation results confirm the efficacy of the proposed algorithm compared to the current fixed-rate update policy. Further, a closed-form solution is obtained for the optimal sampling rate in a network of homogeneous IoT nodes as a benchmark system.