On Stack Overflow, I logged in with Blogger. Now I have an account on another Stack Exchange site under my Google account. Even though, they are kinda the same account, I cannot figure out how to link them.
kiamlauno's answer will allow you to add both logins to a single account. However, if you've already created a Stack Exchange account with each login you won't be able to do that. Instead you'll need to request that a moderator merge your accounts as described in this FAQ post.
You can add the Blogger OpenID account to the other Stack Exchange site, or add your Google OpenID account on Stack Overflow.
Go to your user profile page, and click on "my logins."
On the dialog box that will appear, click on "add more logins...."
The next page is similar to the one you use to log in; it will add a new OpenID account to the ones you already use.