There is such thing as a ghost dragon; I have seen a bunch for my self. I don't know the parents, but the ones you said sound familiar from conversations about ghost dragons. If those are the parents, sometimes it takes over ten times to get the hybrid. Hybrids usually take more than five hatches.
I would check other webistes about Wizard101 for the parents of a ghost dragon, then keep hatching.
They both do, and you don't always get a hybrid pet when you hatch them together, and the pet egg hybrid is colossus and dragon, the ghost dragon is indeed blue ghost and dragon, hope this helps!