Norm, sounds to me like the base assembly, (pins/contacts), are of an inferior quality. The halogen globes run extremely hot & if the hardware ain't up to it.....
The 'black/sooty' appearance means that the globe connections are not tight, eg; loose pins in the ceramic base, which causes a high resistance connection & generates more heat........which in turn will de-temper the metal base pins even more......
Replace the bases & globes, with better quality ones n prob should disappear.
HTH.
L.