Is there a way to hide Matrix blocks based on the Selected Entry Type? I have 2 primary entry types, one for a full width page, and one for a sidebar page. The sidebar page shares about 90% of the same block types as the full width page.  The other 10% do not apply to the sidebar page at all.

Instead of going to the trouble of duplicating (and maintaining) an additional Matrix set for the sidebar page, sans the non-applicable block types, I have decided to use the same Matrix for both pages.

I am looking for a way to disable / hide the unused block types on the sidebar page entry type. Just as with the normal fields which can be assigned or not to the entry types, shouldn't the matrix block types also work in the same manner?

Any ideas?
No. Matrix blocks do not work in that manner. However you define it in the field is how it'll show when you edit it. If it's really important not to show that block type you can write a plugin that checks for an entry type, and hide certain block types based on that result. Granted that's a lot of work for very little gain. You would be better off ignoring the unwanted block types or building a new matrix field.