And how is the shadow market compared to the normal market? More expensive, and it gets targeted every day...
And unlike back market dealers that are mobile persons with distribution chains mining pools are sitting ducks, one letter and the datacenter cuts the pool's servers and that's all. Remember Mega?
One more thing about shadow market. The miner's reward is the sum of differences between output and input values of all transactions included in a block. In this way if all transactions are "black" then all miner's reward is "black" too.
If we imagine that bitcoin will be split to "black" and "white", it's obvious that "black" will be cheaper due to the difficulties to change it to other currency. That's why even if fees in black pools are bigger it will be hard for "black miners" to implement the economic profitability of the process.
Thus, I can't see any conditions for such shadow markets to exist too.