Ultra-black materials, spanning both natural and synthetic systems

Ultra-black materials absorb almost all incident light through the coupled effects of broadband-absorbers and surface architectures that suppress back-scatter and extend optical path length. This coloration has evolved independently across diverse taxa, including deep-sea fishes, birds, butterflies, and other arthropods, where it supports camouflage, thermal management, vivid mating displays, and aposematism. For more than two decades, studies of synthetic materials for optical instrumentation and photo-conversion technologies have advanced independently and only recently biological and synthetic ultra-black have begun to converge. With this convergence, we identify inconsistent terminology, diverse optical measurement setups and lack of standards as factors that limit cross-study comparability. Additionally, we highlight the transition from passively optimizing reflectance toward application-driven designs. To this end, we have organized this review by first defining key optical terms and methods used to characterize ultra-black as these vary across biology and engineering, where overlapping, yet non-equivalent terms are being used. Since there exists no universally accepted standard across disciplines, we propose defining ultra-black as a material whose reflectance (400-700 nm) is below 0.5%. We then turn to ultra-black biological tissues in nature, whose evolved pigments, biopolymer matrices, and hierarchical light-trapping structures can inspire more durable, economical synthetic ultra-black materials. We discuss the various synthetic ultra-black platforms that have emerged since the early 2000s, then conclude with their manufacturing routes, laboratory fabrication, and applications. This review serves to outline the open challenges and future directions for standardized metrology, scalable production, and promising applications.