To "to blither" means to speak foolishly.
A fool is not always talkative; some lack confidence, are very shy and quiet.
When a person's speech is foolish (nonsensical, irrelevant, trivial, unnecessary or excessive) it often gives the impression (possibly false) that they are stupid.
It can be a common trait in people with low IQ, social anxiety (attempting to be likeable), ADHD, or Autism Spectrum,
or can be episodic such as the word-salad of someone experiencing a psychotic episode.
Not all talkative people are foolish. Some are just lonely because they spend a lot of time isolated, say while living on a farm. They get over excited when they meet people and don't realize they're going a bit over the top. Even so, they are practical, good at solving problems, and usually kind and decent - the opposite of idiots.
