I can't change the label of multiline blocks

Why is it not working?

It shows: "expecting the number of inputs to match".
(I know that even if it didn't return an error,
it wouldn't do anything anyway, it's just an example.)

Is there any workaround?

metaprogramming doesn't yet support line breaks inside block labels.

Oh well. My guess is that this is because line breaks are marked as _ (which stands for input) despite not being input.