Do you want to remove the ‘/blog’ slug from post url for the default wpmu blog .
In the sub directory style install of wpmu, ‘/blog’ is added to the permalink structure,category base and tag base for default blog(or main blog).
Removing it is quiet easy. If you don’t wish your posts to have url like http://yoursite.com/blog/somepost-goes-here and want it to look like say http://yoursite.com/somepost-goes-here,I have an easy solution for you.
Download my plugin “Remove /blog slug plugin for wpmu“ , It is has only 10 lines of code.

How to use it.
1. Download the plugin wpmu remove blog slug if you have yet not downloaded it.
2. Install it as any other plugin.You may upload to wp-content/plugins or just use plugin installer to upload and install it.
3. Go to Dashboard->Plugins(Installed plugins)
4. Activate “Remove /blog slug plugin for wpmu”

5. Go to Settings->Permalink
6. Copy your current permalink and then change something in permalink, save it. Now put your old permalink and save again. (See,  you have to save it twice, we are just forcing wordpress to update the permalink structure by changing it, if you don’t change, the permalink is not saved, so It will not work. To get over it, we change it to something else, then save and then revert back to the original structure)

7. Go and check your posts now,they will have no “/blog/” slug anymore.
I have tested it with wordpress Mu 2.8.4a, wp 3.0 multisite but I hope it should work fine with other versions too. I even tested it on a buddypress+wpmu installation and it just works fine.

I will love to hear back your feedback. Please let me know your suggestions/views and anything you want here .
Have a great day!
Brajesh