For some mysterious reason, the WhatsApp Messenger devs decided to remove the default blank chat wallpaper from the last version (2.12.11) released today on the AppStore. Don't know why they do this, because many people don't like all those themed wallpapers that can really clutter your chat message windows.
Anyways, I managed to recover the default blank white-yellow ish WhatsApp stock wallpaper from the previous .ipa package (this is the reason why I always keep some old versions of important apps)
I also adjusted the lightning for a cold / warm view.
The first one is the default stock from WhatsApp ipa original package. The second and third are adjusted by me: