Particle motion blur is actually the result of varying the opacity and the length of particles based on their speed. To accomplish this requires coordination between material assignment and the settings in the particle systems.

Follow these instructions:

• Use the Particle MBlur map in the material that you assigned to the particles. For best results, assign it as an opacity map.
• Make sure that the particle system, PArray, PCloud, and Super Spray, or Spray, supports the Particle MBlur map.
• Choose Rotation and Collision rollout Spin Axis Controls group Direction of Travel/Mblur option.
• In this same group, set the Stretch spinner greater than 0 to stretch the particles as a percent of their length based on the particle Speed setting.
• Use the correct type of particle. MBlur works on all particle types except Constant and Facing.
  NoteInstanced objects with multi/sub-object materials cannot be image motion blurred.