To talk foolishly at length —often used with on. (Merriam – Webster)
To talk for a long time in a silly or annoying way. (Cambridge Dictionary)
Talk in a long-winded way without making very much sense. (Oxford Dictionaries)
If someone is blathering on about something, they are talking for a long time about something that you consider boring or unimportant. (Collins Dictionary)
Talking a good game: how Gareth Southgate has risen above the blather.
Russia Dismisses US Election-Meddling Indictments as ‘Blather’.
Sales Motivation: Ignoring the Negative Blather.