X-ray detectors for space astrophysics missions are susceptible to noise caused by photons with energies outside the operating energy range; for this reason, efficient external optical blocking filters are required to shield the detector from the out-of-band radiation. These filters play a crucial role in meeting the scientific requirements of the X-ray detectors, and their proper operation over the life of the mission is essential for the success of the experimental activity. We studied thin sandwich membranes made of silicon nitride and aluminum as optical blocking filters for high-energy detectors in space missions. Here, we report the results of a multi-technique characterization of SiN membranes with thicknesses in the range from 40 nm to 145 nm coated with few tens of nanometers of aluminum on both sides. In particular, we have measured the X-ray transmission at synchrotron radiation beamlines, the rejection of ultraviolet, visible, and near-infrared radiation, the amount of native oxide on the aluminum surfaces by X-ray photoelectron spectroscopy, the morphology of the sample surfaces by atomic force microscopy, and the aging effects under proton irradiation.