While con or bluff is a perfectly good definition of "blag". I'd have thought it has a slightly more precise meaning within that - to get something for free to which you're not properly entitled, as in: "I managed to blag my way on to the guest list" or "I 've blagged us a couple of tickets for the royal box." I accept that you could perfectly well substitute con or bluff in either of those cases - it's just that both words are capable of being used in a wider context, as well.