blocks and threads
I want to know if blocks in c / cocoa run on a seperate thread to the main thread. Would they be useful for executing computationally expensive code while leaving the UI responsive?
Blocks are just snippets of code bundled up into a callable object. How they run is entirely up to the code that calls it.
Running blocks on a separate thread is not only possible, but is precisely the reason the blocks concept was introduced. It exists to support Grand Central Dispatch, which hides a lot of the complexity of concurrent programming behind a task-oriented model.