Hi @FattyBull
There are probably going to be two components to this–shaping the cloud, and generating a smoky appearance to the surface. Generating the cloud shape can be done conventionally with any of ZBrush’s sculpting tools. You could even just clone a series of spheres and fuse them together for a cloud with nicely rounded features. Brushes that might be useful:
Blob
Form Soft
Magnify
Chisel Shapes
Contrast
There are a number of tricks you can employ to generate a smoky appearance for the surface of the cloud once it is shaped. You will probably get the best results by layering a number of different effects and approaches with a combination of brush based sculpting and more procedural effects.
To give you an idea of how I might do this, I have applied a simple smoky texture to a sphere, subdivided it, and used the Masking> Mask by Color feature to apply that texture as a mask. Then I use a combination of deformation sliders like Inflate, Inflate Balloon, and Contrast to pump up and inflate parts of the surface that aren’t masked. Once you create this kind of variation in your surface, you can use other features like Cavity Masking to deepen or expand certain parts of the mesh surface, but not others. Noisemaker can also be used instead of, or in addition to the above, to generate a series of noise applications at various sizes to enhance the effect.
I spent very little time on this. The more elaborate and smoky the form to being with, the more convincing it will look after you alter the surface. The more manual detailing you are willing to do, the more you will be able to improve the results.
Good luck!