I've used this in the past, it's a very similar example as you need:
<lookFor>[^?]*/Blogs/someUsername/(.*)</lookFor>
<sendTo>~/Default.aspx?tabid=xx&BlogId=yy</sendTo>
If you have blog id in the url above:
http://www.something.com/Blogs/vladan/Default.aspx
it translates it to:
http://www.something.com/Default.aspx?tabid=xx&blogid=yy
and another rule:
<lookFor>[^?]*/Blogs/(.*)</lookFor>
<sendTo>~/Default.aspx?tabid=xx</sendTo>
which:
http://www.something.com/Blogs/Default.aspx
translates to:
http://www.something.com/Default.aspx?tabid=xx
if you will please notice the "Default.aspx" at the end, if you don't have a physical directory called Blogs on the disk, which has a child portal default.aspx page, you will have to have default.aspx (or any .aspx page name) at the end... simply because if you don't have the directory, IIS doesn't know where to map that request (to which handler, aspx or not).
So, either... you create for every sub url a physical directory, and copy the child portal Default.aspx (think of how child portal creation works... it wouldn't work either without putting "Default.aspx" in the url if it didn't create that directory in the web root) and then you can call both:
http://www.something.com/Blogs/Default.aspx
http://www.something.com/Blogs/
or use just the rules, and you can only call:
http://www.something.com/Blogs/Default.aspx
I hope this is clear enough (I am writting it before my coffee :))