That aside though, the theme with the six (possibly seven) "original" derived Keyblades from the X-blade which may be older than all others is loaded over and over with symbolism.
First is the number 6 itself, which in mythology stands, among others, for the term perfect balance, perfection itself, in christian mythology it stands for the six days of creation, the number of wings of the highest order of angels (Seraphim) and the devil/Satan (who was originally a Seraph himself).
In Buddhism, th number 6 even stands for the universe itself in its entirety, as in the sum of the four cardinal directions as well as top and bottom.
Then there are the animal motifs themselves, while they do not correspond exactly to the animals associated with them in classical mythology (toad, snake, lion, snail, pig, goat and peacock)
, they can be associated with the seven deadly sins as except the unicorn and (surprisingly as MX has it) the ram, any of the animals present has had a negative mythological connotation throughout history.
1. The Snake (Anguis) => associated with the devil, also represents the sin of envy.
2. The Fox (Vulpes) => in the middle ages also associated as a messenger of the devil, represents Cunning and Fraud, thus can stand in for the sin of avarice/greed.
3. The Leopard (Leopardus) => Symbol of cruelty and the antichrist, could stand in for the Lion to represent the sin of wrath.
4. The Bear (Ursus) => Symbol of wild and untamable strength as well as insatiable hunger, could represent gluttony.
5. The Unicorn (Unicornus) => One of the two not having had any negative connotations in mythology, as it represents purity, but can be associated with sloth for a) this sin not fitting with any other animal and b) unicorns normally do not do much and sleep often.
6. The Ram (MX's Keyblade, 6th Follower?) => This is a truly interesting case, as in mythology the ram represents the "leader of the herd" who stands above the others, thus could stand for the sin of Pride.
Now the only sin missing is lust (which is in mythology represented by the goat), and the End of Pain-Keyblade apparently has elements of a goat, so if we count it as the seventh, that would take all into account.
To return to the symbolism of 6 though, a possible better "match" would be the Arishadvargas, the six evil passions of Hinduism, which include
1. Kama (Lust)
2. Krodha (Anger/Wrath)
3. Lobh (Greed)
4. Matsarya (Envy/Jealousy)
5. Mada (Pride/Selfishness)
6. Moha (Deep emotional Attachment/Obsession)
As for the blue eye, was it ever confirmed that it is called the "eye of darkness"?
If not, I would bring forward the assumption that instead of representing darkness, it actually may represent foresight, parallel to the Foretellers.
This of course begs the question as to why the 6th Follower's keyblade (if MX's is really that one) has it too since the 6th didn't get a tome.
However, it is said in the story premise of X[chi] that there are Seekers of Darkness disguised as Guardians of Light. Therefore I believe that there actually existed a sixth Union which was headed by the 6th Follower, but this one was a secret society which didn't act openly but infiltrated the other Unions in order to gather as much information about the tomes as possible. The other Foretellers may not even have known that this sixth Union existed nor that it was headed by their former colleague.
Since there were apparently five volumes of the tome, it isn't too hard to imagine that the 6th Follower somehow learned much about the content inside them by various means.