A fuller is a groove forged or milled into the flat of a blade to remove weight while keeping stiffness, borrowed from sword making. It is not a blood groove. Nothing about the geometry vents anything, and the nickname is modern folklore rather than a technical term.
Removing material from the middle of the flat, where it contributes least to stiffness, lightens the blade without letting it flex much more. It is the same structural 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 it is mostly cosmetic, which is why fullers on small knives are usually a style reference to larger ones.
The folklore claims a fuller lets air in and blood out so a blade does not stick. There is no mechanism for that: suction is not what holds a blade in tissue, and a groove that stops short of the tip could not vent anything anyway.
If a listing says blood groove, it means fuller. Same feature, worse name.
A well-placed fuller barely affects stiffness because it removes steel from the neutral axis. A badly placed or excessively deep one can weaken a blade, which is a manufacturing question rather than a property of fullers.
On production knives fullers are almost always machined rather than forged.