why would “dust” collect inside a cap? it shouldnt, or you would be interfering w/ the contact points of the rotor to cap. usually dust buildup inside the dist. is signs of 2 things-- it is usually a reddish/brown color when the bearing is on way out. black dust is usually when there is plastic to plastic contact-- i.e. black rotor to black cap.
thats been my experience w/ pretty much every honda distributor. im sure there are other problems/ideals though.