Not all katanas have a blood groove, which is also known as a fuller or a bo-hi in Japanese. A blood groove is a long, narrow groove that runs along the blade's surface, typically on both sides, and is often associated with the aesthetics and functionality of swords.

The primary purpose of a blood groove is a matter of historical debate. One common misconception is that it is designed to allow blood to flow out more easily when a person is struck with the sword. However, this belief is mostly a myth. The groove is not intended for collecting or channeling blood.

Instead, the primary functions of a blood groove in sword design are to reduce the weight of the blade without compromising its structural integrity and to enhance the overall balance of the sword. The groove removes excess material from the blade's surface, making it lighter and improving its maneuverability. It also affects the distribution of mass along the blade, shifting the center of gravity closer to the hilt, which can improve the sword's handling and control.

While blood grooves are commonly seen in many historical and traditional katana designs, it is not a mandatory feature. There are examples of katanas without a blood groove, particularly in older styles and certain schools of sword making. Some modern interpretations or replicas of katanas may also omit the blood groove for various reasons, such as aesthetic preferences or to stay true to specific historical examples.

In summary, while blood grooves are a characteristic feature of many katanas, not all katanas have them, and their presence or absence does not fundamentally affect the functionality or authenticity of a katana.