Ok what's a Darkmarket?
A place where you can sell illegal contraband anonymously.
I believe it is more about the anonymity than the illegal aspects. You can very well buy/sell legal things but remaining anonymous.
It will unlikely be used for that unless it is for tax evasion otherwise you dont need to be anonymous.
Its the same thing that got bitcoin its big start.
If this is implemented, and with the bounty as large as it is now, it will be, expect gains of x10 to x100.
And that is lowballing it.
Research darkweb if you dont know what it is