I thought blether was Scottish. I certainly heard it a lot from my mum and other Scottish relatives. They’d often talk about having a wee blether.
My cousin who was born and grew up in Scotland lived in Swansea for a while and worked at the DVLA in an office. She used to do a word of the day and everyone would try and guess what it meant. Some examples....
Scunner Simmit Wee besom Spurtle Ben the hoose
I’ll have to ask her for some more. My grandparents were really Scottish.