How can I use two separate gravatar profiles for each blog or is it not possible with one user?
You have to create another username account and transfer one blog to it.
Every blog has a sole owner and it is the person with the username account who registered the blog under that username account and associated email address. No matter how many blogs are created under any username account there will be only one gravatar and one primary blog that can be linked to that username.
In order to completely disassociate blogs currently registered under the same username account and associated email address proceed as follows.
Log out of WordPress.com and clear your browser cache and cookies.
Without logging in to WordPress.com, register another username account under different email address.
To transfer a blog from one username account A to another username account B you must first log in under the username account A that owns the blog you want to transfer to the other username account.
Then you add the other username account B as an Admin of the blog.
adding users http://en.support.wordpress.com/adding-users/
user roles http://en.support.wordpress.com/user-roles/
Then you A can use this process to transfer the blog to that username account B instead http://en.support.wordpress.com/moving-a-blog/#transferring-your-blog-to-another-user-or-account