Spicebush Swallowtail (Papilio troilus) 2006-05-21 Parvin State Park, NJ - Only the Spicebush underside has one spot that is not orange.

Of all the dark Swallowtails, only the Spicebush underside has one spot that is not orange. The point may show a hint of orange from one individual to another, but won't be mistaken for an orange spot.

I don't know how accurate the information at this site is (meaning I haven't researched/confirmed it myself) but it's a nice comparison of some of the dark swallowtails.
http://louisiananaturalist.blogspot.com/2009/06/four-dark-swallowtails.html
another page of comparisons:
http://www.butterfliesathome.com/swallowtail-butterflies.htm