Did the crash increase their business as the economy tanked, or did they suffer as well? It might make sense that a bad economy drove more people to speakeasy's and alcohol, but at the same time people had less money to spend. Overall, was the stock market crash good or bad for the mafia et al, or did it have little effect?
As a follow-up to this, in general, how vulnerable are black markets to economic lurches such as this? Does operating outside the law afford some insulation, or are the effects worsened due to complete lack of regulations and protections?
One aspect that you've overlooked is that the money available to enforce the Volstead Act and other Prohibition laws sharply declined. The Prohibition Bureau had to cut agent's per diem from $6/day to $5/day. James Doran, the commissioner of Prohibition, estimated that he needed $300 million to fully enforce the law, but only received a relatively meager $12 million. (Though according to him, the fines and penalties paid in from enforcement outweighed the costs. In nine years, $141 million was spent but collected $460 million by his calculations. Nevertheless, funding was drying up as the budget shrank.) Enforcement efforts of course suffered.
In some ways, however, Prohibition never really worked, and even before the crash, alcohol use was in some areas growing regardless. The Wickersham Commission reported as such: "as things are at present, there is virtual local option." The Hutchinson News in Kansas (a dry newspaper) said that there was ten times as much drinking after Prohibition than before. Both reports are from after the crash.
Source:
Last Call by Daniel Okrent