www.alphadictionary.com

To obtain something by guile. I'm interested in the origin of this, I gather British word. Perhaps because of the illicit nature of the activity, there are I guess, lots of colloquial alternatives and blag is itself probably in that bracket. I daresay that several regional alternatives could be forthcoming from all around the United States. Any ideas? I wonder also if these various words actually belong to a time as well as a place - fashionable for a while and then replaced by a new one.

I don't remember ever hearing that word. Nor reading it either.

It's very common informal slang here (UK).

Urban Dictionary has one decent answer:
'To convince by rhetoric; to gain acceptance or approval through persuasive banter or conversation; trickery; keenly persuasive; to scrounge by means of conversation.'

You could blag your way through an interview for a job you were perhaps unsuited to apply for. You could blag your way into a club that might not normally let you in. You could not blag some cash from a safe - that's just robbery, you need charm or similar means of persuasion to blag.

I've always wondered whether it is related to blaggard (i.e. blackguard), on balance I think not, blag is light hearted and mischievous rather than dark or mean spirited, but I could be wrong.

So, something like finagle?

Something very like finagle, yes. To me the difference would be that finagling might encompass something more illegal, say an element of swindling, than blagging, which would generally stay within the law, more wheedling while employing charm.

My interpretations of these colloquialisms are mine alone!

Finagle here has no sense of criminality. Rather it's more like cleverness, the ability to work around obstacles to achieve a desired result. The character Radar in MASH was a master at the finagle, cutting through military red tape and bureaucracy to acquire what his colonel or medical staff needed. Admittedly, he would sometimes bend or desert the truth to get the desired result, but he always managed to finagle around and produce the desired result, often equipment in short supply.

Hmm, they could be fairly close then. My sense of a degree of swindling and dishonesty is however upheld by wiktionary (and I didn't edit it honestly guvnor! Though it would be a good finagle if I did alter it to suit my point ):
http://en.wiktionary.org/wiki/finagle

Perhaps a finagle is also a little more elaborate than a blag? I think we need a definitive order of precedence - I hereby suggest:
blag
finagle
scam
short con
long game