Well, obviously it's not a magic fix for everything. But it does sound like an improvement to the economy.
From how it was explained, the black market addition reduced overall silver drops to make up for the money going into the black market. So it really only serves as an item sink. There is going to be an endless stream of silver coming in from mob drops, but the black market didn't increase that any. So assuming they've done well enough to balance the silver faucets and sinks before the black market there isn't any concern there.
They said they would have a certain percentage of items lost in the process that goes up as the price goes down (ie, bought on black market and not dropped), plus you have at least some items that simply won't be looted by anyone and decay - I have seen people just leave gear that they don't have a use for laying about, even T6 stuff. So it does more than just force transfer of items, it actually acts as an increasing item sink as the price of something falls.
In addition to you have the other stuff they have to help prevent market crash on craftables (study and salvage), it puts an effective floor on how low crafted item values will drop. It may get to the point that you have to spend crafting focus to make any profit as a crafter, but it's not going to become totally worthless.
From what I have seen the crafted item market is very insulated against crashes. And the low end raw resource market stays afloat because they're all needed to refine the high end resources.
The market that seems very prone to crashing is that of farmables, especially lesser used stuff like chicken eggs. I witnessed some very very illogical prices for chicken eggs on several occasions. I remember at one point the price of chicken eggs was selling for 2 (and not just a few, hundreds and hundreds). I believe you get around 30 eggs, but to do that you have to feed them 10 food. And all the food was 100+. So people were feeding them food worth at least 1000, harvesting the eggs, and selling it all for 60. Even when the price was 10 they weren't worth the feed. One problem there is that there's no way to convert the eggs back into the veggies - you can't feed the eggs to any animals or use them in place of vegetables in recipes or use any kind of salvage system to get a portion of their value back. And the other problem is everyone can have an island, yet not everyone has the economic sense to know when to produce something and when to stop producing. People grow crops and raise chickens and sell the eggs on the market on some kind of autopilot without paying attention to market prices and realizing they would be better off just to ditch the chickens and sell the crops. I don't understand it because to me the economics of not producing something if it costs you more to make it than it's worth comes naturally, but I know that for a LOT of people (including many college graduates) that just doesn't click. So they go on producing eggs at a cost of 1000 and selling them for 60 and flooding the market even more.