I was having the same problem. All dice results show as plain black. No successes or Face huggers. Stress dice show as black too. So, as Wes said: The dice images are hosted on the Github Roll20 Repository, and are a part of the character sheets roll template so they are available to all levels of Roll20 users." Now it is suddenly working properly.
My guess is that the Github was unavailable for a few hours in the middle of the night and the images defaulted to a black square. I'm not a coder but that's my guess. Since all previous rolls show correctly now. My guess it was just missing images and not broken math. Also thanks Wes, for making this cool character sheet.