The new (correct) system for navigating a multiblog is ALMOST there. But one flaw remains. When you enter the multiblog via the aggregator homepage (in my case www.lalaurette.com.ar/blog), and click on a post, the "canonical" blog is not set IF THAT POST ONLY HAS ONE CATEGORY, so the click takes you to the particular blog it belongs to, and the navigation is consistent from then on. But I want to see the posts for ALL blogs! That's why I entered the aggregator!

So:

MULTIBLOG (AGGREGATOR): http://www.lalaurette.com.ar/blog/

BLOG A: http://www.lalaurette.com.ar/blog/index.php/emporio

BLOG B: http://www.lalaurette.com.ar/blog/index.php/planeta

This post (1) is in blogs A and B, but its main category is in blog B: http://www.lalaurette.com.ar/blog/index.php/planeta/kirchner

This post (2) is only in blog A: http://www.lalaurette.com.ar/blog/index.php/emporio/2010/11/13/trazos-huellas-sombras

You enter the multiblog and click post 1: You stay in the multiblog (you can tell because the skin doesn't change).

You enter the multiblog and click post 2: You are taken to blog A (the skin changes). You no longer see the posts in other blogs unless you go back.

In my opinion, if you entered via the aggregator, the only time when navigation should change to a particular blog is when you click a category. At all other times, you should stay in "Blog All".