Conservation of momentum demands that the aileron roll is around the body axis (more precisely: the longitudinal axis of inertia). If only an aileron input is made, the result is a rolling moment (plus a bit of yawing from adverse yaw) but no change in pitch.
Proof: Do an aileron roll once without a bit of pitching up before you start aileron input and then with. In an airplane with a regular cambered wing, you need to increase pitch to compensate for the wing's incidence and zero-lift-angle, or your loss of lift will be substantial when flying inverted.
Of course, in an F-16 all pilot commands are interpreted by a computer and will result in movements of all control surfaces, so a pure aileron roll is probably impossible.