I've recently encountered this issue, trying to modify infill to 100% for a select area. Our problem is the same though.
When you place the blocker, the overlap settings shows seven parameters highlighted orange with values of 0. Leave them there, don't be tempted to remove them like I did. These parameters set the size and thickness of the walls and roof/base of the virtual cube you are adding. With these original parameters it will not draw this solid box.
You can see the thread here if you wish to see where my problem and solution lies. https://community.ultimaker.com/topic/38846-adjusting-default-settings-in-per-model-menu-bar/
I hope that helps, and in return, could you explain how you got the fuzzy-blocker to work? I can't get it to work here. I have fuzzy enabled as a global option, but when I added a blocker to a part, added the five fuzzy parameters to the blocker and had them turned off and set to 0, it still wouldn't work for me to get the part to print normally as a smooth-walled object.