It means you have a perfectly normal dragon. I'm no expert, but I reckon if they're all dark then they're either pissed off, frightened, or trying to absorb heat.
You've got nothing to worry about if it doesn't stay dark for long.
Watch this space for pics.
DONE! Sorry these were taken on my camera phone, but you get the idea.
Before: (this is one dragon when he was asleep. He was perched up against the glass, which made it convenient to shoot)
After: When handling this dragon I noticed these mad colourations. I don't know if these two are the same dragon, but it comes and goes.
On an interesting note I opened my curtains in my room wide open yesterday. A sharp beam of sun hit the tank. One immediately went all dark and agro. I've noticed this happen when I take them outside. I think it's just because they're not used to being outside, and tend to freak out a bit.
