I always had the idea that dragons weren't really "discovered" - I think they were kind of just a given presence in ancient times (like dogs, only a lot more dangerous to have around). But as the population decreased they fell out of general knowledge, and after a few centuries they became the stuff of legend.
As for the uses of dragon's blood, I think all the general theories have been covered. I only have one to add - an element in the Sorcerer's Stone. It's an idea I got when I saw the movie version of SS (I know that's not really what I should be going off of) and observed that the stone was a dark red, like blood. I figure if blood was used in its making then it would probably be the blood of some powerfully magical creature, like a dragon. However, if this use is true, then Dumbledore wouldn't have been the one to discover it; it would have had to have been Nicholas Flamel.
Dragon keepers like Charlie Weasley would of course have to do the usual jobs of making sure the muggles don't see the dragons, and keeping enough animals around for the dragons to eat. But I feel like they would have to protect the dragons as well. There must be dragon poachers in the wizarding world, people who hunt dragons for their magical parts like blood, not to mention their hide. Bill Weasley is said to have been wearing boots made of dragon hide when Harry first meets him in GoF (this always struck me as odd - how could Bill wear bits of dead dragon in the presence of Charlie, who clearly cares a lot about dragons? Wouldn't Charlie get mad at his brother?), and he can't be the only one. And if dragons do get killed, some might leave behind eggs or young dragons, which the keepers would probably have to raise in captivity for at least a few months, until they could survive on their own.
Dragons in Harry Potter are most certainly vicious and not of human intelligence. I guess they had to be that way in order to suit JK's purpose in the first challenge in GoF, but I've always liked stories in which dragons are considered wise, like Eragon, or even evilly intelligent, like The Hobbit. And personally, I think it would have been pretty exciting if Harry had had to get the golden egg from a dragon that could shout threats and insults inbetween breathing fire.
Some very nice ideas. 5 points.