The answer to this lies in the blog site definition. Open this file:
C:\Program Files\Common Files\Microsoft Shared\Web Server
Extensions\14\TEMPLATE\SiteTemplates\Blog\Xml\onet.xml
This file is what deploys all the lists and each of their associated views or pages. If you scroll down to the Modules section, you'll see all the web parts being added to each of these pages. Look for the following node inside each page declaration:
<AllUsersWebPart List="x" WebPartZoneID="BlogNavigator" WebPartOrder="x">
This is where the blog-specific navigation (which is a web part inside a zone with the ID "BlogNavigator") is being pushed into the left-hand navigation area of your masterpage. So if you open one of the blog lists' view pages in Visual Studio, e.g:
C:\Program Files\Common Files\Microsoft Shared\Web Server
Extensions\14\TEMPLATE\SiteTemplates\Blog\Lists\Posts\Post.aspx
You will see that it is replacing the default contents of the "PlaceHolderLeftNavMenu" content placeholder in your masterpage with its "BlogNavigator" web part zone. To add the default current navigation above this, merely copy and paste the existing menu declaration from the masterpage above the blog's "BlogNavigator" web part zone declaration.
It goes without saying, that whenever a developer says "copy and paste", he really means "cut out the existing code, place it in a usercontrol, and reference that usercontrol in both locations". This way you only have your left hand menu declared in a single place. But you knew that already...