Delta fibers (aka slow pain fibers) carry sensations of pressure, pain, and cold if I remember correctly. Applied pressure can "crowd out" slow pain conduction. What's the first action you take after stubbing your toe? You rub it. It's the same methodology that chiropractors use with "manipulations" very rarely is anything actually "manipulated" but rather a quick applied pressure occurs at the injury site which does the aforementioned small digression, but pertinent. Anyone who has experienced severe back pain can attest to an almost miraculous resolution of pain. Termed the "Gate theory of pain", while nerves can convey multiple signals at once, we process them singularly, meaning the "gate", or your processing of sensory data, has been confounded at the gate.
Apply this pressure to a stab wound and you'll likely get the same result. Unfortunately your fast pain fibers large and well myelinated (think of a large and well insulated conduit) has its own specific pathway that is difficult to occlude. Thus the victim may have felt the initial sharp stab of pain of the dagger, but would have been spared (and so confounding the victims perception of the event) the dull aching pain that would typically follow the initial sharp jolt of pain. Had the guard winced briefly (albeit they are likely trained not to, but physiology is a bitch) it would have made the entire event more plausible. That said, it is very possible that the guard could have stood for another hour with the pressure signals from his belt occluding the dull pain signals that were likely present, but not received so to speak.
There is also evidence - this is slightly tangential but again relevant - that we tend to receive the signals we are familiar with rather than ones we are not, creatures of comfort and habit we are. I.e. the guard was quite familiar with the pressure sensation of his belt while on duty - that makes sense to him - therefore the dull pain signals are being fought on 2 different fronts: 1. A real physiological mechanism, and 2. His own minds hard wiring to gravitate towards familiar sensations rather than foreign ones.