Evidence like blood at a crime scene is classified under:

Blood at a crime scene is classified as trace evidence because it involves small quantities of biological material that can be transferred from one location to another. Trace evidence refers to materials that are not readily visible but can provide significant information about a crime, such as hair, fibers, and bodily fluids. This type of evidence often requires specialized techniques for collection and analysis, as it is typically found in small amounts and can be crucial for linking a suspect to a crime scene or victim.

While physical evidence encompasses a broader category that includes any tangible items found at a crime scene, trace evidence specifically denotes the more minute and specific types of materials that can be crucial for forensic analysis. Other classifications like hard evidence and legal evidence do not accurately describe the nature of blood at a crime scene, as hard evidence generally refers to more substantial, often item-based evidence, and legal evidence pertains to admissibility in court rather than the type of physical characteristics of the evidence itself.