Yeah, because they are both godlike beings and one of them is the ruler of their own world, it makes sense that they would have some knowledge about the Keyblade, especially since Genie has been around for many years.
Its just the ability to see Keyholes, but correct! The sealing part goes along with the Keyblade's powers.
All Keyblade wielders obtain two things when they undergo and pass the bequeathing ceremony: the ability to see Keyholes and they eventually get chosen by a Keyblade.
If you remember, Riku and Kairi were the first ones to arrive on the island before Sora when the storm hit; Riku probably told her he opened the door, and she went to try and close it.
Ventus had nothing to do with it. Sora became a Keyblade wielder by accidentally taking Riku's Keyblade on destiny islands. Then at Hollow Bastion the Kingdom Key officially chose him as it's true wielder.
