One who is a master at extending a conversation for hours without saying anything useful. Beyond a casual blatherer, a blatheroso typically engages in considerable pontification and avoids reason, but will mistakenly feel they are the only one capable of making sense in any conversation. Also, while a common blatherer will flit from one pointless thought to another, a blatheroso will drag out one topic for a grossly extended time.
That proselytizing, religious friend of yours is a real blatheroso. I didn't think we'd ever get out of that creation "discussion".