Since I was a child I had always wondered about these "blue brick streets" and while on my last visit to Old San Juan I asked my mom. Mom told me that the cobble stoned streets in Old San Juan are paved with these beautiful blue stones called adoquine. They are blue stone cast from furnace slag which were brought over on Spanish ships as ballasts. The characteristic blue color comes from age and moisture.
Becuase of their age, the Adoquines you will see around Old San Juan are cracked, faded and worn down due to high automobile traffic and foot traffic. What a shame, these streets are part of a long Puerto Rican Heritage and they should be preserved by the closure of streets to automobile traffic.