Block tab on Model Options dialog under Block Display header.
You can choose to switch on/off display of external walls for each block. Generally for standard extruded blocks it is best to show external walls to help reference locations within the block with respect to existing openings etc.
For Pitched roof blocks and General block forms the external walls can obscure the internal space and so the default for these block types is to hide the external walls when at block level in the modeller. You can override this default behaviour using this option.
Note: It can be useful to switch off external wall display when drawing partitions in roof blocks.