It is basically sucking air in from one side and this sucks dust with it.
While you’d think that this would mean the dust is on the “back” of the fan blades, it is mostly on the front because they are designed to “hit” the air that comes in out toward the front.
Not all the dust sticks to them though and they push a lot of it through. Hence, you see that dust accumulates inside of a desktop pc case fairly quickly despite being a mostly closed off container.
TL;DR:
sucks in a lot of air which has some dust in it and the blades hit the dust causing the dust to stick to them.
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