Colloidal gels made of carbon black particles are "rheo-acoustic" materials: their mechanical and structural properties can be tuned using high-power ultrasound, sound waves with submicron amplitude and frequency larger than 20 kHz. The effect is demonstrated using two experiments: rheology coupled to ultrasound to test for the gel mechanical response and an ultra small-angle X-ray scattering experiment coupled to ultrasound to test for structural changes within the gel. We show that high-power ultrasound above a critical amplitude softens carbon black gels at rest due to the formation of micro-cracks in the bulk. The gel softens exponentially with a characteristic time of a few seconds. High-power ultrasound also eases the flow of the gel thanks again to the formation of micro-cracks. High shear rates however tend to select a gel structure that is less sensitive to ultrasound.