By default, Blog.com displays some ads on sidebars, but you can add your own by subscribing the Disable Ads / Custom Ads upgrade.
Besides removing completely our ads from your blog, purchasing this means that you’ll be able to place one widget (or several) with an your own ads and monetize your blog.
Start by purchasing the premium feature in the Upgrades section of your blog’s dashboard.
Once the upgrade is active, you’ll be able to place your ad widget in your sidebar(s) or widgetized footer.
Go to Appearance > Widgets and drag this widget to the area where you want to place your ad.
Then, insert the code provided by Google AdSense (or similar service) and save it.
If you not have an account with AdSense, you can always place a simple image with a link to your website or any other thing you’d like to publicize.