In rifle shooting, "point blank" means that you know the trajectory of the bullet and sight in so that by holding the rifle sights on the desired impact point you know out to a certain range the bullet will hit above or below or on the position the sight is being held on. For rifle hunting deer for example, If the kill zone of the deer is either 3" above or 3" below the center of the chest behind the shoulder, you can hold right in the center of the chest on any distance out to your point blank distance and know you will get a "kill" with out worrying about how far you are shooting. All you have to know is what that point blank distance is. In the example above, as you shoot beyond your point blank distance, the bullet will drop below the arbitrary 3" used. To my knowledge this is mostly a hunting technique so you don't have to figure a hold over or under in order to make a shot into the "kill" zone.
I'm not sure that there is much applicability in archery, unless using a sight.
Arne