We don't have synonyms for blethered yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are lettered, tethered and bothered.
(n) :
(especially Northern England, Scotland, Northern Ireland) Alternative spelling of blather. [(derogatory) Foolish or nonsensical talk.]
(v) :
(especially Northern England, Scotland, Northern Ireland) Alternative spelling of blather. [(intransitive, derogatory) To talk rapidly without making much sense.]
(n) :
(especially Northern England, Scotland, Northern Ireland) Alternative spelling of blather. [(derogatory) Foolish or nonsensical talk.]
(v) :
(especially Northern England, Scotland, Northern Ireland) Alternative spelling of blather. [(intransitive, derogatory) To talk rapidly without making much sense.]