How to load custom php file in Wordpress themes

I'm just created a website using wordpress and a custom theme for it. This is my theme directory

/themes/customtheme/
 -index.php
 -header.php
 -footer.php
 -blog.php
 ....

The index.php includes header,footer,information to shown on my website, and a link to my blog page.

Blog file will looks same ask normal blog site.

may i know how to wite a hyperlink(a href) in index.php under theme directory so that it can prompt to the blog.php when click?