I wonder: does "blag" rhyme with blog or flag? I'd like to use that word.
I'm assuming 'flag' but unfortunately the British slang dictionaries that I'm seeing so far don't offer much in the way of pronunciation help. There's also a chance that these slang terms, like many other words, vary in how they're pronounced based upon subtle regional variations, just as many words have regional pronunciation differences here in the USA.
I think that in this case it might be best to seek the help of one of our many British FReeper friends, unless a better online British slang dictionary can be found than the ones I'm seeing so far..
It ryhmes with fag.