Abstract: A blender for blending a mixture of flowable materials, such as the carrier and toner components of a developer for an electrostatic processor, comprises a rotatable chamber having radially angulated passageways extending through its periphery for guiding the materials into and out of the chamber and internal axially angulated plates for cross-mixing the materials. The passageways are open, but the mixture tends to at least temporally dwell in the chamber and is, therefore, subject to a tumbling action, because the radial angulation of the passageways exceeds the angle of repose of the mixture. The internal plates impart an axial component to the movement of the tumbling mixture to provide cross-mixing of the materials.