I need to setup a page that will show me a selection of all blog posts tagged with a certain tag, let's say "mytag"

I could do it with a redirector, but then the URL changes to /blog/tag/mytag

What I need is a Meta Blog functionality that will show blogs tagged on my site, not on Technorati.

Any ideas?

Well all you need is a function which returns a set of blogs with a given 'tag' so its fairly straight forward. Im pretty sure this will work for you. Its pretty crude. I better way would be to have an exact match search rather then a LIKE.

function ArticlesByTag($tag) {
return DataObject::get("BlogEntry", "Tags LIKE '$tag'");

Then in your template you can do <% control ArticlesByTag(SomeTag) %>

Great stuff willr, I guess it's the first step towards getting dirty with the internals.
I will look up the documentation on how to do that kind of hacking in order to preserve the hack through upgrades.
I appreciate the help, keep up the good work.

This is not a hack at all. If you put this in your mysite/code/Page.php it will not be effected by any upgraded. You can even override the default blog template for your own by creating a folder in themes/ called 'yourtheme_blog' with the same folder / file structure as blog/ (but without code). This keeps all your code modular and free from any upgrade issues