Most of this has already been covered in the posts above. Thanks for that. 
A few additional thought on this matter:
Textures: A pure entry should be more that just textures. But using them usually isn’t a problem. (Of course, creating them yourself feels more pure than just using/downloading pre-made materials.) [And, even though the rules don’t care about this: always check licenses before using stuff from the internet. You don’t want to get sued over a weekend challenge entry.]
Addons: I think, Generator-Addons (e.g. sapling) which come with blender, can be considered being part of blender.
External Post processing: Minor changes shouldn’t affect the pure/open category. Applying the full range of G’MIC effects (which are absolutely wonderful) is a whole different story.
As it has been noted before, it is hard to come up with an exact definition that works for every case but isn’t to restrictive at the same time. As in toxicology, the dose makes the poison. (However, in this case the poison isn’t very dangerous as entering open entries is absolutely fine.)
I think this sums it up quite well: If you can honestly say to yourself, “Yes, I mainly used Blender to make this”, “Yes, nearly all of the foreground models are new”, “Yes, most of the models are new”, then the entry is Pure.
(I like to read “foreground models” as “important parts of the entry”.)
This might help too: Instead of overthinking the rules in a hairsplitting way, just keep in mind the intentions behind them. Try to be fair. Be open about how an entry was created. This way we can keep this event being fun - and hopefully everyone can learn a thing or two. As far as I can tell, calling this a ‘challenge’ instead of a ‘competition’ was a brilliant/wise decision.