One thing that might surprise a lot of people about Philadelphia is that we have a HUGE vegetarian population, and a long history of veggie friendly restaurants that sort of fly under the radar. Blackbird's location right off of South Street is actually the former home of Gianni's Grill (now closed)—for a long time one of the few places in town to get a vegetarian cheesesteak. Blackbird opened 2 years ago, an all vegan Philly-style pizzeria and sandwich shop.
More gas-deck slice shop than high-end pizzeria, the crust on Blackbird's double wide slices is better than most slice joints in Philly. Sturdy but not tough and chewy, and nicely charred. The red sauce is also a nice departure from the usual Philly sweet stuff, more like roughly crushed tomatoes, applied sparingly. Even the vegan sausage on the South Philly (also topped with eggplant, broccoli rabe and shaved fennel) was pretty good.
But the fake vegan daiya "cheese" they use is definitely an acquired taste. It must be heaven for vegans who haven't had dairy in 5 years, but to me it tastes like something you would find growing in the corner of your refrigerator. I feel like their pizza would be phenomenal without it, but then again If I were vegan I might travel halfway across the country for something like this. They do offer a few pies without the fake cheese, not available by the slice when I was there. Great crust, sauce and fresh toppings, but if you aren't vegan, it's probably worth it to custom order a whole pie sans daiya.