That's right. There never was a B service prior to the Chrystie St. connection. The BB came into being in 1940 along with the D and F lines when the 6th Ave. line opened.
The 6th Ave. line was supposed to be a four-track line right from the beginning. Why that didn't happen right away is another story. Prior to 1967, the express tracks at 34th St. were used by BB trains which terminated there during rush hours, and F trains when they didn't run to Broadway-Fafayette Sts. There is a maze of scissor switches between 34th and 42nd Sts., including TWO sets of scissor switches between the express tracks.
The express tracks were finshed and ready for use when the Chrystie St. connection opened in November of 1967. However, because the new 57th St. station wasn't quite ready yet, the express tracks were used only during rush hours at first by B and D trains. During other times, the D ran on the local tracks south of 34th St., then switched over between 34th and 42nd (and north of W. 4th St.); the B terminated at W. 4th St. during non-rush hours when it was running. The 57th St. station opened on July 1, 1968, at which time the B began terminating there at all times except rush hours. At the same time, D trains began running express along 6th Ave. at all times, while B trains ran express only during rush hours initially.
During the water main break in 1962, CC service was suspended, and the AA and BB swapped hours of operation while keeping the same terminals. Trains marked DD ran during non-rush hours via 8th Ave. and made all stops; D trains ran during rush hours along the same route (express in Bronx peak direction, but only between Tremont Ave. and 145th St.; express in Manhattan between 145th and 59th Sts.). C trains ran during rush hours between Bedford Park and 34th St.-6th Ave., running express along CPW.
There was a C express route from 1933 to 1940 which ran during, I believe, rush hours. It ran from 205th St. to Jay St.-Boro Hall and was dropped when the 6th Ave. line opened.
The water main diversion marked the only time the DD marking was ever used. The FF marking was never used.