Like the other darker swallowtails, it has blue, white and orange markings, and very similar and related patterns on the top and bottom-side of its wings.
I would love to tell you all the places in Maryland I have seen black swallowtails, but I really can’t be sure. In flight, its extremely difficult to differentiate the black from the spicebush and pipevine and even if I get a shot, I usually need to studdy the markings closely to make a correct assessment.
I took the photo above at Eastern Neck Island Refuge, so I have definitely seen one there. I believe I also photographed a Black Swallowtail at Dan’s Mountain WMA, and possibly at Idylwild WMA.