Taking a blade offworld is problematic assuming they follow the same rules as living spren. I personally think that some are kept safe in shadesmar by their spren relatives, some were lost beneath the accumulating crem(drop a shadeblade and a couple highstorms latter no one will find it), and that some are owned by secret organizations(like Nale's Skybreaker's, the ghostbloods, and the Shin Shamanate).