Do event Loot Boxes (Summer Games, Halloween, Winter Wonderland, etc...) only contain limited time event items or can I also get normal items from event loot boxes?
The Overwatch blog has a post about the Summer Games.
Regarding loot boxes, it says:
Starting today (and for the duration of the event), any new Loot Boxes you earn or purchase will be sporting an updated look, but the packaging isn’t the only thing that’s changed!
Inside, players will have a chance to find a variety of new cosmetic items commemorating the Summer Games—including themed sprays, emotes, victory poses, highlight intros, player icons, and some very special skins. Though they can't be purchased with credits, each seasonal Loot Box you receive is guaranteed to contain at least one item from our Summer Games 2016 collection.
While any Summer Games item you unlock will be yours forever, the collection will be heading back into the vault on August 22 when the event concludes. So, if you’ve got your eye on something specific, there's no time to lose!
I interpret this to mean that all loot boxes obtained during the event will be "Summer Games Loot Boxes", and that each loot box will have a mixture of normal and summer games equipment.