After consulting with the Stokes Field Guide to Birds/Eastern Region, I can confirm that I did indeed sight two large Black Vultures soaring above the rain-swollen Perkiomen Creek earlier this week. Based on the appearance of these large black birds with pale whitish primary feathers, short tails, and appearance similar to the Turkey Vultures that I see quite regularly, I probably should have been able to ID them as well. We had ruled them out as being Turkey Vultures, though, with their easily recognizable naked red heads and totally black bodies, but hadn't really gotten a good sighting of the Black Vulture's less distinguishable but larger naked, wrinkled, gray heads. Based on the Stokes Guide range map, these vultures were at the northern limits of their normal range, which might explain why I'm more likely to sight Turkey Vultures, who are quite abundant in my neck of the woods.
Thanks for your assistance, Rose.
~~Leequi