Someone (I'm not sure who) is sitting on a couch playing with a Dalmation dog. The dog is romping and friendly but then it morphs into a black, Great Dane sized canine figure in silhouette. end.

I know dogs can symbolize obedience, loyalty, and companionship. The Dalmation seems to fit that bill, but this other thing?

The metamorphosis of the dog probably stands for a change in your life. The fact that the dog changed from a solid animal to a silhouette could represent the way the future is less substantial than the past and present.
Also, if you think of black as representing the unknown, or the mysterious, the fact that the dog got blacker--from black and white to all black, could also signify the way the future is more intangible.

Interesting. I do view black as a color signifiying the unknown. The black and white dog represents things set in stone or "black and white" issues. The new dog is the unknown. No wonder it freaked me out!