If it's neither blocking nor async then it's a completely regular function and you don't even have to call it with spawn blocking, there's nothing that prevent calling a normal function from an async one.

And in the opposite situation, if you call an async function then you are doing IO so your function must be either async or blocking, there's no third way in this direction, so when you're doing IO you have to make a choice: you either make it explicit (and thus declare the function async) or you hide it (by making a blocking call).

A blocking function is just a function doing IO that hides it from the type system and pretend to be a regular function.