http://clang.llvm.org/get_started.html
http://compiler-rt.llvm.org/
Blocks are supported for programs developed for Mac OS X 10.6+ and iOS 4.0+,[1] although third-party runtimes allow use on Mac OS X 10.5 and iOS 2.2+.[2]
Regardless, both of you appear to be correct:
BlocksRuntime - a target-independent implementation of Apple "Blocks" runtime interfaces.
http://clang.llvm.org/get_started.html