Had I been able to figure this out, this image may have been my "sky" entry

First, since this is such a high altitude pov, how does one fade blue sky to deep blue or black as altitude increases?  In bryce or post?  How do you do it either way? 

second is for opinions only.  option 1)  high altitude plane, perhaps SR 71   2) high flying, high tech globe circling type ballon...  this is the way I'm leaning..     or  3)  on the sinister side, a bomber flying away fm mushroom cloud. ....   my last choice personally

sky is fm sky lab manipulations plus two additional sky planes, two volume spheres, and camera inside a glass sphere to give curvature. 

I really need help on fade to black thing.