A fuller is a long groove forged or milled into the flat of a blade to remove weight while keeping stiffness. It is a structural feature borrowed from sword making. It is not a blood groove, and it has nothing to do with blood.
Removing material from the middle of a blade’s flat, where it contributes least to stiffness, lightens the blade without letting it flex much more. It is the same idea as the web of an I-beam. On long blades like machetes and swords, the weight saving is significant.
On a four-inch knife the effect is mostly cosmetic. The blade is too short and thick for the weight saving to matter, which is why fullers on small knives are usually a style reference to larger blades.
The folklore says a fuller lets air in and blood out so a blade does not get stuck. There is no mechanism for that; suction is not what holds a blade in tissue, and a groove that does not reach the tip would not vent anything anyway. The term is a modern nickname, not a technical one.
If a listing calls it a blood groove, it means a fuller. Same feature, worse name.