The χ-blade appears to be made up of two crossing Kingdom Keys, with the teeth facing outward. It has a filigree that slightly resembles one on the Ultima Weapon, and the blade slightly resembles the Dream Sword.
It should be noted that the incomplete χ-blade's coloring is more red-orange than bluish-yellow, and the blade is slightly covered with an aura of darkness. It also seems that the incomplete one appears to have chips in the side of the blade, and it lacks a part of the filigree on one side, and part of the Kingdom Key's hilt, that makes it look somehow broken or shattered. The Keychain's two crossed Kingdom Keys is reminiscent of the symbol of the "Keys to the Kingdom of Heaven". Although it is not seen in this incarnation, it is especially significant that the Kingdom Key has a silver blade and gold hilt, and the Kingdom Key D has a gold blade and silver hilt, matching the keys on the symbol. When the χ-blade hits a target, the released symbols are gold and dark blue stars, symbolic of light and darkness.
The Keychain is a black and red heart that is similar to the Heartless symbol, only without the fleur-de-lis bottom and jagged cross, with two Kingdom Keys crossed over it. The chain starts out as two separate chains that form into one.
Although χ-blade is pronounced "Keyblade", the letter "χ" is the letter chi from the Greek alphabet. This is why the name of the weapon is spelled "χ-blade", but, as Master Xehanort explains, can be pronounced "Kye-blade". Master Xehanort explains to Ventus, with a visual demonstration of the ancient letter "χ", that he is not referring to a Keyblade that Keyblade Wielders use. He continues to state that the letter itself carries a meaning of "death" and that it "spells endings".
There's not really much that can be pulled off from the design standpoint of the χ-blade. To me it just feels like a giant deus ex machina that wasn't really needed. When I first heard about it while playing through Kingdom Hearts: Birth by Sleep, I was rather indifferent. We already have the Keyblade of People's Heart to be the polar opposite of the Keyblade. So why was it needed? Personally to me, it just looks like really bad fan art. (Reading the Wiki again, materializing Kingdom Hearts is a good reason, but still not by an actual design standpoint.)
Altogether it is supposed to be different, so why make it something that feels like it was just using recycled assets? You have two Kingdom Keys, with two handles and I suppose that the guards is where you're supposed to grab on the weapon? Then there's the symmetrical aspect to it that resembles the Ultima weapon. Finally the Keychain it self is just two Kingdom Key's crossed with a the Heartless emblem without the fleur-de-lis bottom.
If the χ-blade is supposed to be something where a pure light and darkness meet, shouldn't it have something that resembles that? We have the 13 Darknesses, and the 7 Lights, however I couldn't find anything within the design that could give any alliteration to this idea. Looking at it is supposed the perfect union, so the symmetrical part they achieved was correct, but that's all there is to it really. It has the power materialize Kingdom Hearts, so shouldn't it be inspired by Kingdom Hearts in a way? (The 13/7 Shards/Fragments is an exception, but their is no actual representation physically that is noticeable on the weapon)
Later, we find that χ in χ-blade is greek for Chi. Which in Cantonese/Mandarin is pronounced with the soft sounding chuh sound. However the Japanese equivalent is Ki (Think Naruto VS Dragon Ball Z). So basically it means, spirit or life in a sense When Master Xehanort says that the χ is meant for death, due to the fact that it can end ..."things"... I'm pretty sure Sora has used his Keyblade to end things as well. Perhaps when Xehanort meant death he meant the death of all who participated in the Keyblade War.
I know I am arguing over a topic where in hindsight. We're attacking with giant keys, but hey. I just feel like this was rather lazy.
