You can refine several blackboxes at once in a mutual whitebox. That might be useful if these building blocks intensively interact, cooperate or provide similar interfaces.
You find an example in the diagram below: On the left part of this diagram you find a whitebox (level-1) which contains two blackboxes
Both of these are refined in a single whitebox on the right side of the diagram. Consistently, all interfaces of level-1 are contained in level-2.
The origin of the refined (level-2) blackboxes is given by prefixes.
Apply this tip with care, as it violates the clean hierarchical decomposition usually applied in the building block view.