First comparison run. Setting the particle size to 0.5, then applying the blur effect modifier to different amounts. Seems ok for making particles smoky, but notice the shadows are still obvious, and that the blur sometimes distorts objects behind the particles. High blurs can also reveal the particle polygons inside. Shader in this case is the default gray, and particles are on diamond shape setting.

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Barbequed Pixels?

Your friendly neighborhood Wings3D nut.
Also feel free to browse my freebies at ShareCG.
There might be something worth downloading.