People unaware of the "Fours Theory" likely never visited The Ansem Report back when it was, shall we say, working. The Fours Theory is basically a group of theories based around the four "Realms" mentioned in the extra pages of Ansem's Reports. The Keyblade theory is based around the theory that each of the "four" (There are at least five -- if not six.) main Keyblades comes from a different Realm. While there is also another theory in the group that says that each of the Keyblade Masters originated from a different Realm as well, that one really doesn't hold water with the new information that's surfaced since the theory's original creation. And because of that no one should try to place a Keyblade with a Keyblade Master simply because the Master came from a certain Realm. For the most part, this would refer to the BHK -- however, it could also refer to King Mickey in certain situations.
The origin of the theory aside, the currently known 'blades are the Keyblade of Light, the Keyblade of "Dawn," the Keyblade of "Dusk," the Keyblade of Darkness, the Keyblade of "Hearts," and the "Brave Keyblade." Three questions are likely to arise from that -- it can be answered with only two answers, though.
1) What are the Keyblades of Dawn and Dusk?
The Keyblades of Dawn and Dusk are more commonly called the Keyblades of Present and In-between. In-between is known as "Twilight" these days, to clear that up. While most don't consider Present are part of the "Twilight," in a sense it is. Since Present is neither pure light nor pure darkness, it's a combination of the two, therefore placing it "in between." The Keyblades from these two Realms likely belong to Sora and Mickey -- the gold on their Keyblades could be used to assign them names, if you prefer. If that confuses you, think of the gold part as the sun -- the "setting sun" (golden handle) could be called Dusk, while the "rising sun" (golden blade) could be called Dawn. Get it now? ... Hopefully you do.
2) What is the Brave Keyblade?
The "Brave Keyblade" refers to the second Keyblade (Oathkeeper) Sora gets when in his "Brave Mode," as shown during the in-game parts of the trailers. Whether or not this is another new Keyblade, or one of the other Keyblade Master's Keyblades (Mickey's?) is unknown. Hence why I said "five or six Keyblades."
The Brave Keyblade and the Hearts Keyblade (AKA the "Heart Unlocker" and the "Princess Heart Keyblade."), from what we've seen of them, seem to be situational Keyblades. Meaning, of course, that they apparently can only exist when certain requirements have been met. Because of this most people don't deem them as "true" Keyblades. While saying that they're artificial may be pushing it a little, they really aren't quite the same as the others.
As for the original poster's doubts about Riku being the Keyblade Master of Darkness: While it's true that Riku is technically using both light and darkness now, he still only wields the powers of darkness. The "light" aspect, in my opinion, was more along the lines of him using his dark powers for "good," and not so much that he actually has the ability to use both. It should be noted as well that Riku didn't use "light powers" against Ansem -- he used his own "Personal Darkness" against him. Since we're on the topic anyway, it may be worth noting that BHK has yet to wield either light or dark powers as well. Therefore deeming him "Twilight," aside from the presence of Oathkeeper and Oblivion, may be jumping to conclusions prematurely.
Eh... and just because it always seems to happen: No, I'm not claiming any of that as fact. It's just me restating things that have been said here and elsewhere combined with my own personal view of the information.
EDIT:
- Removed font and font colors...
@Riku the wicked: ... And you couldn't have just PM'ed me or skipped it?
