Ok I think I found a solution for you
So I set up blogs on my test forum, made 3 entries, created a supermod and did some testing
If you do not have Blog Categories you are going to have to create them. If you do have Blog Categories then you need to make some decisions based on the solution.
Assuming you have blog cats (or just created some either way

) go to acp vbulletin blog / blog cat perms ... edit super moderators and set both options to NO.
EDIT to add ... make sure you select "use custom permissions" up on top of the page.
Any blog entry in that category will not be visible to super mods.
Now, if you already have blog cats but don't want all blogs in a particular blog cat blocked to super mods ... you're going to have to move those to a different blog cat.