The nickname doesn’t get used much at all any more, but back in the day, the source code trunk was called the Blue Line.
Where did it get that name?

From the of the whiteboard pen.

When the branching structure was worked out, the trunk was drawn with a blue pen. If you were in that meeting, and you wanted to raise a point about the diagram, you might say,
“But when the red line meets the blue line…”,
“How do changes get from the green line to the blue line?”

Everybody called the trunk the “blue line” in the meeting, and that nickname carried forward into the internal documentation.
Of course, if you weren’t at that meeting, it was a giant mystery why the trunk was called the Blue Line.

A mystery that has now been resolved,
long after everybody stopped using that nickname.

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