Carbon black is a common polymer additive that is used for reinforcement and for its ability to enhance physical properties, such as conductivity. This article pertains to an X-ray scattering (SAXS) study of a conductive grade of carbon black and carbon black-polymer composites. The scattering pattern for such blacks displays a surface-fractal-like power-law decay over many decades in scattering vector q. It is often assumed that small-angle scattering from carbon black aggregates can be described in terms of surface-fractal models, related to particles with fractally rough surfaces. Such self-similar surface roughness is usually easy to identify by microscopy; however, electron microscopy from these blacks fails to support this assumption. It is proposed here that this apparent surface-fractal scattering actually represents a more complicated morphology, including overlapping structural features and a power-law scaling of polydispersity. One use of conductive black-polyethylene composites is in circuit protection devices where resistive heating leads to a reversible association of carbon black aggregates that controls switching between a conductive and a nonconductive state. Scattering can be used as an in situ tool to observe the morphological signature of this reversible structural change. Scattering patterns support a model for this switching based on local enhancement of concentration and the formation of linear agglomerates associated with the matrix polymer＇s semicrystalline morphology.