The black dot most likely represents the terminal with the black silkscreen outline on the PCB, the designers wanted people to make sure 'out' pin gets special attention and demarcation.
I've done this for service people in documentation to make sure they 'don't screw up' polarity (and sometimes they still do)
It also appears that the terminal is even a different color for this purpose .
If you didn't plug the common terminal of the heaters into 'out', it could be really bad. It probably won't matter as much if you got BA and BR swapped.