Can I make a content type for a single user's blog entries? I would like site announcements to appear in a block on the sidebar.

I can do that with a content type, but Admin has a blog and I was wondering if/how to automatically give all blog posts of Admin (and no other user) be of a content type
I think what you want is to create a view. Show all blog entries by 'user 1' only in your block. 
Create a view: Under your display(on the left) select block from the drop down. Give it 
a name, say Test1.
Under Fields add Node:Title and Node:Body.
In the filters add Node-type:Blog, Node:Published:Yes and User:Name=Admin

Then goto Site building->Blocks and you should see Test1:Viewname. Enable that block and configure it to show it on the page where you'd like.

Hope that helps.