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 Foo and Bar.

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.

whitebox with two blackboxes refined to single whitebox

Apply this tip with care, as it violates the clean hierarchical decomposition usually applied in the building block view.