Currently, no matter which dragon you kill (with the possible exception of the Master Dragon, I haven't checked) the chat announcement always says "xxx has slain the dragon."
I think it would help clarity to say exactly which dragon was slain. So the announcement in chat would say "xxx has slain the Fire Dragon." or "xxx has slain the Master Dragon."
I'm aware that everybody has access to that information while the dragon is up, but the moment it dies, so does any indicator about which dragon it was except for checking the enemy team's buffs.