How to insert flash intro page into my blog?

Intro pages are stupid. So is meaningless Flash.

http://www.dougv.com/blog/2007/01/14/stupid-web-site-tricks/

That said, you need to indicate the blogging platform you are using for this question to be answerable.

If you are using WordPress or other blog platforms that you installed on your own domain, then your blog's first page is index.php.

All you need to do is create your flash intro page and name it index.htm (or index.html). This file extension has priority over .php so your flash intro page will show up before your blog's first page. You can either add a "enter" link on your flash intro page to your blog page, or you can do a redirect or page load command after the flash to redirect your visitors to your blog page.