I am a student of the Seido karate fighting style, I would like to know why weapons training starts only at the black belt level?
Sorry if this is a vague answer, and I don't know the first thing about the Seido karate style, but I remember an instructor (I can't remember what style. Kajukenbo, perhaps?) at a martial arts camp once telling me that they only taught weapons at black belt level to avoid weapons bias.
The idea was that a practitioner should be fully proficient in empty hand fighting before learning how to handle a weapon. If you first learn to fight with a weapon, and then got disarmed, or dropped the weapon or something during a fight, you might not be able to make the switch back to empty hand fast enough if your mind is still fixated on the weapon. Or you might get so focused on weapon fighting that you neglect other perfectly good striking opportunities during the fight. The classic example of this is in the movies where two people fight over possession of a knife/gun/weapon completely ignoring the fact that they could have just let go of the weapon and beat the snot out of the bad guy in the time it took them to recover the weapon.
He also said that once they do start weapons training, the weapon would just become an extension of their normal fighting style.
As for Seido karate, I think you'll have to ask a Seido karate instructor.