Actually, we did vacuum the attic as thoroughly as possible before putting in fiberglas insulation in 2006. (Previously there had been no insulation whatsoever.) But with all the additions to the house, there are so many nooks & crannies that it was impossible to get all of the dust. And I suppose the walls were full of it, too.
It doesn't sound like there's much more I can do about the black dust except to keep dusting and hope it eventually subsides.
I thought I was doing the right thing by ventilating and insulating the attic, but I almost wish I'd let well enough alone. After all, the house got along fine without the vents for over 100 years.