Ah, now I see where the confusion is coming from. The bearing blocks do NOT move -- only the nut (well, the nut and whatever you use to attach the nut to your gantry, which is not one of my bearing blocks).
You need two bearing blocks per axis. One is just a radial bearing, typically mounted at the end opposite the motor, which only constrains the screw radially. The other is the "stack" of radial and thrust bearings -- this keeps the screw from moving radially and axially -- remember, there is an equal and opposite force on the screw to whatever thrust you are putting into the gantry. This stacked up bearing block absorbs the thrust load -- otherwise it would all be on the motor bearings, which is definitely not a good idea.
Hope that helps,
Ahren
www.cncrouterparts.com