This paper investigates the problem of blind detection of orthogonal space–time block codes (OSTBCs) with quadrature amplitude modulation (QAM) in multiple-input multiple-output (MIMO) systems over quasistatic flat Rayleigh fading channels. To resolve the inherent rotational ambiguity in blind OSTBC detection, we propose a structurally constrained QAM constellation that enables unique symbol recovery without the use of pilot signals. Building on this design, we develop a low-complexity iterative detector, referred to as the iterative maximum-likelihood with averaged initial channel estimate (IML-AICE) detector, which jointly estimates the channel and transmitted symbols. The proposed detector incorporates a novel initialization strategy and an iterative refinement mechanism inspired by clairvoyant maximum-likelihood detection, leading to improved convergence and detection accuracy. The proposed framework enables reliable blind recovery of OSTBC symbols, thereby improving spectral efficiency by eliminating pilot overhead. Simulation results demonstrate that the proposed IML-AICE detector consistently outperforms existing trained and blind detection schemes across a range of signal-to-noise ratios and system configurations at low computational complexity.