If by aio you mean Posix aio - on Linux it's implemented with user space threads and blocking I/O. Posix aio on BSD systems is implemented as kernel space thread (aio_write/etc are syscalls on BSD, and glibc functions on Linux).

If you mean io_submit, then yes, but in vast majority of cases, actual `io_submit` syscall will block, because of metadata updates, unaligned reads, etc ...