The exquisite diversity and functionality of biological materials is truly remarkable, especially since they are composed of a small set of abundant chemical elements. While engineering materials primarily require specific, often unsustainable, chemical compositions to realize their functions, nature achieves unparalleled functionality through optimized architectures that span multiple length scales. Water, with its ubiquity and unique structural dynamics, plays a pivotal role as a nanoscale "working fluid" in shaping the properties and functionality of nature's materials. Here, I will introduce a novel class of sustainable, interactive materials that derive their functionality from the interplay between hierarchical structures of hard matter and water. I will show how these " Blue Materials " can mimic natural processes such as water-driven mechanical actuation, capillarity-driven water transport, and humidity-responsive coloration and light scattering. I will also highlight their potential for innovative applications, including electrical energy storage and generation (hydrovoltaics), thus extending the functionalities observed in nature.