I'm in a Clan War and I see a little mini dark elixir storage at the top of my Clan Castle, but I don't have an actual dark elixir storage. Will the dark elixir stay in the Clan Castle until I can claim it?
Loot won in a Clan War is automatically stored in the Clan Castle at the end of every war. This includes Dark Elixir. War loot cannot be spent until it has been collected, and if you collect the loot without a Dark Elixir Storage you will lose it.
The Dark Elixir will stay in your Clan Castle until you collect it, or until its raided. I recommend you read this page from the wiki which gives you more detailed information on Clan Castles.
The miniature gold, elixir and dark elixir storages on top of the clan castle represent where the war loot is stored after a war has ended. You can see these in the following picture of a level 2 Clan Castle:
The Dark Elixir will stay in your Clan Castle until you are raided. It will not go away, and if you win Dark Elixir in your Clan Castle the next war, the Dark Elixir will only accumulate until collected. In other words, it won't disappear or be wasted unless you are raided or try to collect it without a Dark Elixir Storage