Small-angle X-ray scattering (SAXS) and transmission electron microscopy image analysis experiments on a series of commercially available carbon blacks verify recent computer modeling results on scattering by mass fractal aggregates of globular particles. We show that the derivative of I(q) vs q SAXS data demonstrates a crossover point ge between surface scattering by primary particles and scattering by mass fractal aggregates. This crossover point is related to the mean diameter and the size distribution of the primary particles.