I have applied voronoi fracture to a model and some weird blue faces started showing up, the second model (building) has themeverywhere. Any idea what these are and how to solve it? is it a modeling problem? is it normals? … I have made a short video of the problem:

Those are backfaces, meaning the normals are pointed inwards, not outwards. It's a viewport visualization that is active while you're in SOPs.

Thank you, how do I sort it out? I'm new to Houdini. I can still it at object level.

some normals are probably reversed in the original model, see attached.
try using ‘orient polygons’ in facet sop to fix it.