The Silvers claim that a Blue coerced/seduced/raped (depending on who you ask) a Silver female, then the Blues killed her when she tried to defend herself.

The Blues claim that a Silver female played up to a Blue male then killed him for the specific purpose of stealing his blood.  She attacked the male's family when they discovered her with the body and was defeated by them.

Blues are renowned for their prowess in battle, and so the Silvers wanted to take their blood and use it to enhance their own.  The Silvers are known for their cunning.  


So now the Silvers - or, at least, their bloodsworn - know about the existence of a Blue half-dragon.  

It would not surprise me if there were many Silver halfbloods: when the Silver take a Complement, they sleep with that person.  If it's a male mage with a female dragon, the subsequent offspring is strongly draconic.  If the male mage shows any signs of attempting to claim the child and raise it himself, he is instantly killed.  If it is a female mage with a male dragon, she will almost invariably die during childbirth anyway.  

There is one woman who survived, barely, and though she is only in her forties, she is weaker and more frail than a woman twice her age.  She knows the truth of the Silvers and the truth of the conflict, but she is bloodsworn, and she takes her oaths seriously.  Even though the dragon she was Complement to took a new Complement after she proved too frail to work magic any longer, her oath is her bond.

The Silvers have their bloodsworn train their halfbloods in magic, and marry the halfbloods to each other.  A draconic halfblood is stronger in magic than any human mage, made stronger by the fact that their human half is already an incredibly powerful mage.  They share the draconic resistances to normal fire, and can, with time, be taught to use draconic senses.  They essentially are a dragon who cannot take dragon form (though they don't breathe dragonfire; they cast it).  The dragonfire of a halfblood is twice as strong as a normal bloodsworn's dragonfire (but only half as strong as a true dragon's).

The Silvers view this as an advantage.  Daniel has it partially wrong: most mages when face to face with a dragon in human form or a half-dragon would only sense the presence of very powerful magic user.  They likely wouldn't be able to distinguish the difference.  The bloodsworn are taught to differentiate; being one all his life, and mostly isolated from other mages, he doesn't know they can't tell.  So a halfblood can move about as a normal human unless confronted by a bloodsworn, another halfblood, or a full dragon.

Dragons are essentially in hiding from humankind, because of the fear and hatred humans have for them.  The Silvers dislike this need to hide and are resentful of humankind.  They plan to use their halfbloods to take over human kingdoms, one by one, until they can essentially rule over human civilization.

The Blues are regretful that humans fear them.  They would rather simply be left alone to live their lives and let humans live theirs.

Anyway!

Since the Silvers have such cunning plans for their own halfbloods, the existence of a BLUE halfblood is worrisome to them.  They might think the Blues are doing what *they* are doing, and so they would necessarily view the extinction of Riona as a necessity.

Alternatively, a Silver halfblood might suggest bringing her to their side.  If he and the Blue halfblood mate and produce offspring, those offspring would have the Blue blood that they have been wanting for centuries.  So either the Blue halfblood must produce children for a Silver halfblood - not for a full Silver, since she may not survive childbirth, and they want her to have many children - or she must die.