I've looked through the Jenkins docs, and I can't find this answer.
I'm played around with it, but my results seem to be somewhat inconsistent.
Is the "block on upstream builds" option transitive?
For example, suppose Project A triggers project B and project B triggers project C.
If A is running and B is triggered, B will not run until A is finished.
If A is running and C is triggered, will C wait for A to finish?
Yes it is transitive.
From the jenkins option description:
When this option is checked, Jenkins will prevent the project from building when a dependency of this project is in the queue, or building. The dependencies include the direct as well as the transitive dependencies.