That would have to be determined at the discretion of the implementer of the library. Rust is after all capable of being used to write code that will run in a context where there is no kernel and thus no syscalls.
The Rust standard library's block_on uses a global ThreadPool, and the docs recommend using a LocalPool if you need finer grained control.
So, to answer your question it depends on the executor (the thing that implements block_on).
So for anyone reading for the correct details: the block_on I was thinking of is part of the futures crate, which is not in std, it's not an "official library".