The "blood groove" is a term commonly used to refer to a groove or groove-like feature found on some katana blades. In Japanese terminology, it is called "hi" or "bo-hi." Contrary to popular belief, the primary purpose of the blood groove is not to allow blood to flow out of a wounded opponent. Instead, it serves several functional and aesthetic purposes:

1. Weight Reduction: The blood groove helps reduce the overall weight of the katana blade without sacrificing its structural integrity. By removing material from the center of the blade, it redistributes the weight towards the edges, resulting in a lighter and more maneuverable sword.
2. Resonance and Vibration: The presence of a blood groove can affect the resonance and vibration characteristics of the blade during combat. It can alter the harmonics and distribution of vibrations along the blade, potentially enhancing its handling and reducing unwanted vibrations.
3. Symbolic Aesthetics: The blood groove is often included for decorative and aesthetic purposes. It can add visual appeal to the blade, creating an attractive pattern and accentuating the curvature of the katana. It is seen as a traditional and iconic feature of Japanese swords.

It's important to note that the blood groove has limited practical effect in terms of blood drainage or functional performance. Its impact on weight, resonance, and aesthetics is more significant. The overall design and craftsmanship of the katana, including factors like blade geometry, heat treatment, and balance, play a more crucial role in its performance and cutting ability.

It's worth mentioning that not all katana blades have blood grooves. Some blades are made without this feature, known as "bo-hi nashi," and they can still be fully functional and effective swords. The presence or absence of a blood groove is a matter of design preference, tradition, and the intended purpose of the individual sword.