Which terminal should I connect it to for a "C" wire when none are labeled "C"?
If you follow the schematic, you'll see that two wires come off the secondary of the transformer; Black/RD and Black/BL.  Black/RD goes to CH-4 (fuse), and comes out at CH-5 as Red, then eventually terminates at R. Black/BL goes to splice CH-2, an splits to multiple BL wires.  One of these terminates at CH-7, and says to see note 3. Since I can't see note 3, I'm assuming note 3 says something about this wire being used as an auxiliary C.