All firearms are set up to shoot the projectiles(s) upward at some degree. Somewhere along the bullet's line of flight, the bullet reaches the top of the trajectory - as high as it will get - and begins to drop. Obviously this distance changes with the velocity and ballistic coefficient of the projectile. The faster and less prone to loose velocity, the further the bullet will travel in a given amount of time, and the 'flatter' the trajectory.
So, one desires a handgun (or rifle, shotgun, machinegun) to have sights aligned with the direct path of the bullet with no deviation in windage. The bullet strike for elevation will vary with distance.
Here's the trick: The elevation variance is negligible within certain distances. (In a military rifle, this is called 'battle sight zero'. In a pistol, it's just the normal sight setting and doesn't have a title.) For instance, a .45 ACP hardball round, fired from a Government Model pistol - if sighted in two and one half inches high at 50 yards, will strike about seven inches below line of sight at 100 yards. (According to the Chuck Hawk website information). For self defense purposes, one does not need to change sight picture to hit a torso sized target out to 100 yards.
A 9x19mm pistol is similar, but not exactly the same. Three inches high at 50 yards will strike just less than two inches low at 100 - for a 124 grain bullet at 1100 f/s.
Before anyone gets all worked up about shooting a handgun at 50 or 100 yards, the point is, a single sight setting will serve for any given loading.
More than that, switching loads will not change the point of impact so radically one's sight setting need be adjusted. This still in the context of self defense, and the target area being the size of the "8" ring of the NRA B27 target - about 12 x 18 inches.
For precision bullseye shooting, adjustments need be made to score X ring hits.
Just for the tally book, the bullet crosses the line of sight - according to the sights - twice; once on the upswing and then again on the downswing. (This is a very useful fact when sighting in a brand new rifle, or a new scope or sights on an old rifle.)
And - just for the tally book - the term 'point blank' refers to the distance a firearm and cartridge will strike a defined size target WITHOUT changing the sight setting. It does not mean "can't miss" range, contrary to semi-literate reporters or television characters.