Currently, a blocking profile does not download or display messages from a blocked profile. Profiles on the same device have separate block settings:
Sub-profile view filters would need a completely different implementation. There are actually already some filterlike search functions, but they are not very discoverable (accessed by typing magic strings into the searchbar), nor are they reusable. Subprofiles could also be useful as (or attached to) an improved filter UI.
There is current discussion of group moderation and specifically blocking within groups. It seems highly desirable to have subprofile-level blocking, especially if some subprofiles are bridged social media accounts from platforms that have more problems with obnoxious users than DC typically does. So discussing this at the same time as subprofiles might be useful.