I have once attempted to quantify the criminal market depth of Bitcoin using the largest marketplace at the time as a point of reference, it came down to a zero with a decimal point in comparison to the actual currency favoured by criminals, USD which actually does have a sizeable chunk of black market in it, just the scamming industry is equivalent to the fifth largest country in size of economy, it mostly resides in USD.
Yes it takes root among rhetorics, yes they have no basis whatsoever for their hypothesis (when have you ever seen one in news? all they do is argue in rhetorics so save us the braincells). First they laugh at you, then they fight you, then you win. Declaring Bitcoin the currency of criminals is no more than a petty reason to justify an attack so we're certainly moving in the right direction in that regard.