A spoiler 'tag' to hide text for questions and answers was added back in December (>!), however, Stack Exchange has yet to add anything similar to chat. Considering chat conversations can lead just about anywhere, a feature to hide text in chat would be very useful.
I propose a system very similar to this plug-in created by @Ardaxi. It hides text enclosed in back ticks (`) with a black bar, and shows it when the cursor hovers over it, like so: