Blackadder was accepted as an allied family CDSNA in July 2012.
According to Wikipedia,
Clan Blackadder is a Scottish clan. The clan historically held lands near the Anglo-Scottish border. Today Clan Blackadder does not have a chief recognised by the Lord Lyon King of Arms, therefore the clan has no standing under Scots Law. Clan Blackadder is considered an armigerous clan, meaning that it is considered to have had at one time a chief who possessed the chiefly arms, however no one at present is in possession of such arms. The arms of Blackadder of that Ilk are blazoned as: Azure, on a chevron Argent three roses Gules.
The clan name is a territorial name derived from the lands of Blackadder in Berwickshire. The lands, in turn, are named after the Blackadder Water, a river which is part of the River Tweed system, and which runs through the Scottish Borders. The name Blackadder is derived from the Old English awedur which means "running water" or "stream". Black, in The Surnames of Scotland (p. 78), states that, “Blakadir de Eodem held the lands in the earldom of March in 1426”. It is curious that Blackadder and Douglas have very similar meanings.
William Anderson's book, “The Scottish Nation” (published in 1877), provides some compelling evidence for linking Blackadder to our Douglas family.
"Andrew Blackadder, the proprietor of the estate, married a daughter of the house of Johnston of Johnston, ancestor of the earls of Annandale, and had two sons, Robert and Patrick. Robert, the elder son, espoused Alison Douglas, fourth daughter of George, Master of Angus, and sister of Archibald, earl of Angus. He followed the standard of the Douglases at Flodden in 1513, and was slain with his father-in-law and two hundred gentlemen of the name of Douglas, on that disastrous field, leaving a widow and two daughters, Beatrix and Margaret, who, at the time, were mere children [Redpath’s Border History.]" [Anderson.1:309]
Anderson, William. The Scottish Nation, Or, the Surnames, Families, Literature, Honours, &
Biographical History of the People of Scotland (1877), 3 vols. Wakefield: Microform Academic, 2001. Print.
Wikipedia: Clan Blackadder. http://en.wikipedia.org/wiki/Clan_Blackadder