I don't think the items in lockboxes depend on whom you get the lockboxes from. I think it's all random. That said, I also don't think it matters what level you open them at. For example, though, if you get a lockbox from the level 60-70 range, if it's an item, it'll be within that range no matter what.
Honestly, I think I may have seen ONE blue in all of the lockboxes I've opened. Speaking of which, I need to pickpocket some more to level my lockpicking (as I haven't really done that since my early 70s.