I think the title speaks for itself but please let me know if more information is needed!
you can’t relabel a vanilla block into a custom block but if you relabel a custom block, you only see other custom block in the list
It would be pretty neat to make the custom blocks more customizable, but in the long run the idea is pretty useless.
maybe there’s a userscript that was/can be made for that, but as far as i know, no
primitave blocks (the non-custom ones) can only be relabeled to similar blocks set by the devs and custom blocks can only be relabled to other custom blocks of the same type in their respective categories and you can’t change which ones you’re allowed to pick from