As the Keyblade Graveyard rather bluntly shows, this isn't the case; Keyblades remain in existance after their masters die (though I assume this only applies if the Keyblade hasn't been put away prior to their master's death) And Keyblades also don't vanish after their wielders have been knocked out, it seems. Look at when Aqua knocked Vanitas out in Neverland; his Keyblade doesn't vanish after he's knocked out. However, I like Grass' idea that Eraqus' Keyblade is a old Keyblade that's been passed down between Masters of LoD. Makes sense and explains its simpler appearance. ^_^
As for Aqua's Keyblade, it's almost definitely because the gap between her and her weapon is literally between realms; She's in the Realm of Darkness, while it's in the Realm of Light.