In most implementations, blocking in an async function has unintended side effect of blocking all other async function running on the same executor.
It's not the case in C#. It is discouraged, but mainly because there just used to be so much sloppily written async code that managed to bring down threadpool to its knees despite hillclimbing and blocked threads detection doing a lot of heavy lifting, so the community has grown scar tissue against this. It's rarely an issue if ever in the last 5 years or so.