These bricks are just shapes that extend over their brick boundaries, right?
No. In each BLB there is a section where you can define how bricks can be placed in the area of the brick. You can choose a different definition for each 1x1F section of that brick.
X = Cannot place bricks in or on top of this.
u = Cannot place bricks inside this, but can on top of.
d = Cannot place bricks inside this, but can below.
- = Bricks can be placed inside of this area.
b = Cannot place bricks inside, but can on top or below.