This is more than a longshot, but I was in Crossroads Bakeshop in Doylestown last week, and one of the owners said he's been trying out some heavy-duty German breads. They're hit or miss right now, as far as when they're there, but at some point they'll be in their regular rotation. Just to give you a grain of hope....
You may want to try Metropolitan Bakery; they have some nice dark, crusty breads, but no schwarzbrot.
I miss going to the Backeri when I was stationed in Germany! For those who've never been there, they are to Germany as cheeseteak and hoagie shops are to our area, one on every corner!
I think the best opportunity would be to go to the Northeast to one of two large markets- Bell's Market or Net Cost.
11701 Bustleton Ave
Philadelphia, PA 19116
8354 Bustleton Ave
Philadelphia, PA 19152
Now, both of these markets are russian/East European oriented but the "chorny xleb" at a russian bakery is going to be very close to the schwarzbrot from Germany.. And the selection at both of these stores is amazing. Also wonderful places to find a wide range of butters, sour creams, pickled fish, and cured meats. Language is the only barrier. Note these are both supermarket size markets, not small delis.