Twitter now has their official follow button for websites. An easy to place script that displays the normal twitter button but this one is not to tweet but to follow. This you can place anywhere that can take html. Following is immediate as long as you are logged in.
When you click the text username that is next to the button you will be able to see the twitter profile and latest tweets of the twitter account.
The code to use is a script that you will paste where you want your button to appear and here it is.
And you can edit this to a dark colour if you blog black here
[UPDATE]: For wordpress.com bloggers you can get an easier process that has even more functionality like text colour edit, button colour plus an option to show a twitter feed above the follow button. This can be done by just a shortcode embedded in a text widget in your sidebar, or just anywhere you like. See more details.