A blagger's just someone who's lying for w/e reason. It's more English than Scottish slang.
Yeah, it really must be your Northern slang on this "blether" shit because if Dot Cotton's a blether I'm officially confused. Because, while Dot rambles on, she doesn't really lie or talk shit. Do you just mean they talk a lot?
And to have a blether just means to have a chat here......plus, Glasgow>you.