So I have various blue and purple equipment bags from dungeons and doing quests for the dark brother hood (eg sithis headpiece equipment bag), will the contents of the bag scale to when I open them, or are they locked onto what my level was when I received them?
Yes, the item level is determined when you open it (and not when you receive it).
So if you gain such a container being champion 120 and wait until being champion 160+, the item level will be champion 160.
Source : my own experience.
Same apply to level below 50 (since you can go to gold coast at any level).