I believe you are mistaken about the blackened bricks. These are clinkers, bricks that were blackened (and often deformed) in the original firing of the clay. In earlier days, when the even heating of brick kilns was problematic, clinkers were common. Clinkers were sold as inferior but cheaper bricks. As you're in San Francisco, I suggest you wander through Chinatown, particularly some of the smaller streets and alleys. You'll find many buildings where clinkers are used as decorative accents.

My house had a chimney built of clinkers, until it started leaning after the Loma Prieta earthquake. It seems the original mortar had been made using beach sand (not uncommon for the era), and the salt in the sand had, over the decades, reduced the mortar to not much more than the original sand. It was only a later repointing that held the chimney together at all. The clinkers from the chimney have been recycled into planters.