How to remove black box from my cloud, cycles First I create my cloud via metaball -> converting it with Alt+C-> pressing generate cloud.

Rendering is ok (it doesn't have this black bounding box), but after some time or some manipulations this black bounding box shows up.

However if I copy everything to new file it's ok again. Can it be something to do with way of creating sky?

Any suggestions?

You can check blend file:
https://yadi.sk/d/BZkRwk6T3DqUda
Your max transparent bounces setting is set to 0 in Properties > Render settings > Light Paths:



This setting limits the number of transparent surfaces a ray is allowed to pass through. Setting this to 0 means rays are not allowed to pass through any transparent surfaces, rendering black instead.

Set this to something greater than 0 (e.g. 8) and all should work as expected.