"Dargavel" was the name of a place in southern Scotland, a county, I believe. Today there is still an estate, with a mansion, in Bishopton, Renfrewshire, Scotland called the "Dargavel House". The county of Dargavel was located in that area (see
http://upload.wikimedia.org/wikipedia/commons/a/ac/ScotlandRenfrewshire.png for a map of Scotland, with Renfrewshire highlighted). However, at the RootsWeb.com database (see
http://worldconnect.rootsweb.com/cgi-bin/igm.cgi?surname=Dargavel&given=), there are many records of people with the last name Dargavel who came, not from Renfrewshire, but South Lanarkshire (see
http://upload.wikimedia.org/wikipedia/commons/3/33/ScotlandSouthLanarkshire.png) and Dumfries (see
http://upload.wikimedia.org/wikipedia/commons/f/fc/ScotlandDumfriesGalloway.png). There were also many Dargavels in Tasmania, Australia and Ontario, Canada who probably immigrated there.
Also see
http://www.bishopton.org.uk/history/old_history/history-dargavel.htm for more information about the place called Dargavel.
My guess is, people who came from Dargavel, Scotland were called "John of Dargavel" or whatever, and, being shortened to "John Dargavel", it became their last name. Hope this helps somewhat...