We're in the midst of determining which buttons should be blue. Here's a good place to talk about it and try to determine how we should use it.
Blue is a progressive color. When I'm editing an article, a blue "Next" button lets me know what to do to move forward in this workflow that I intentionally started.
If I want to use a blue button that isn't particularly in any workflow, say.. I'm looking at a page detailing why this user hid my comment, it has to be an action that moves me forward and completes a certain task from the current page, which could be a button like "Talk to User."
Determine what the page is about and its intention and then think about what moves users forward and completes the user's most likely intended task, or our preferred route (if any) for the user.
Another example.. say I'm at the Wikimedia shop, the cart link would be blue because it moves me forward and out of the site when I'm done with the page (which is to make you buy stuff).
In the case of user profile, which is a page that shows you a user's activity, I wouldn't know what would be appropriate for a blue button. It's a pretty neutral page to me or I just don't know enough about this page to say what should be blue.
mm