The building block view should be a hierarchical explanation of your source code, of the static (code) structure of your system. It contains blackboxes of various level-of-detail.

It can be very understandable to make your important blackboxes part of your topmost refinement level, called level-1 in arc42.

If your important blackbox is something quite small which you don’t want to explicitly contain in level-1, then you could contain it in any refinement level - and mark it in its containing whitebox in color or by other visual means: Draw it larger than others, attach a graphical label/icon to it, give it a colored border - anything to let it stand out. Be pragmatic!

See also the questions in section C-5 (Building block view).