If a project has both "Block build when upstream project is building" and "Block build when downstream project is building" selected, it will only block when downstream projects are building.

Example:
ProjectA -> ProjectB and both have "Block build when upstream project is building" and "Block build when downstream project is building" selected.

If ProjectB is running and ProjectA is launched, ProjectA blocks as expected.
If ProjectA is running and ProjectB is launched, Project B does not block. It does not respect the "Block when upstream project is building" option.