Whenever things in your dream are dark they tend to take on a negative tone. A dark sky means that sadness is surrounding us all, and a dark cloud means that sadness is surrounding the area under the cloud. The dream interpretation of these events is not too far off from the way that they are generally perceived either.
Dreaming of a dark sky can be a good thing or a bad thing depending on how you interpret it and what your general mood has been like lately. A dark sky can mean rain, thunder, lightning, and all of the negatives that come with it. But these can also be seen as positives depending on the type of person you are and what your life has been like.
For example, if your days have been filled with sun and you have just been expending all of your energy as hard as you can at a very fast rate for demanding people, to know that you can calm down along with the rest of the world when you see a dark sky coming your way is a blessing. Most of the time the dark sky is a symbol that it is time for you to stop enjoying yourself and move along. The day is over, or the good weather is over and it's time to pack up and go home. For some people though, this home time is the only time that they actually can enjoy themselves, so the interpretation can go both ways.
If you have a dream about a dark cloud then this means that you are having a good time but there is something that is on your mind which is threatening to ruin it at any given moment. Maybe you're having a great day out at the park with your kids but all you can think about is that case you need to work on when you get home, or you're at a party but all you can think about is homework. You can solve this issue by not procrastinating anymore and instead resolving to just get your work done when it needs to get done.