O.o You know you can get all the information directly from source at NVidia…
Just to make it clear, PhysX stands for the physics model which actuates on elements in your scene (ie. meshes) while Blast (and also APEX was) stands for how the destruction is applied to elements (ie where and how the cracks, slashs, bents, will appear). Once you have the destroyed parts realized (aka debris are defined), then Blast ends its task and then PhysX comes in telling how the destroyed parts will react to a force applied at some point of the mesh. So, each module with its responsibilities.