The voltage on the blue line should be the same as at the battery + and the alternator output stud. That;s what the VR uses to determine the required alternator output. If it is reading low then it will try to run the alternator harder to bring the voltage up. Probably dirty/loose connections in that line. I usually remove, clean, tighten and reinstall them.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.