AFAICT, that isn't blue, it's green and gray. It means that:
Either you were awarded reputation in a forum where reputation doesn't count (e.g. Geeks Lounge)
You were given reputation by someone who doesn't have enough rep power to influence you. As an example, take a look at this member who awarded you reputation in a technical forum but it still showed up as gray.
There is another color, red, which means that you were given a negative reputation (typically given when someone doesn't agree with your post and wants to negatively influence your reputation).