I seriously doubt anyone in the black market cares about being able to prove a transaction took place, in fact I think they would rather the opposite. You and I know the importance of financial privacy and the bad things that can happen by practicing bad financial privacy methods, but most people are oblivious to this. The sheeple of this world that are still using FIAT, and even a lot of Bitcoin users, are oblivious to the importance of financial privacy. Therefore, it narrows the population of possible Monero adopters to the small group of people that understand the value of financial privacy and the black market. The black market is a much bigger and valuable subgroup, and more people within it will be adopting an anonymous cryptocurrency rather than the general public.
I also think it's silly to think that Monero will be used in a large way by "legit companies/institutions." That space is reserved to Bitcoin for the time being. For that to change, Monero would have to achieve greater liquidity and popularity than Bitcoin. I just don't see that happening... ever... because Monero doesn't provide enough of a benefit over Bitcoin for everyone to switch over.
The same goes with Zerocoin, Anoncoin, or Darkcoin. Anonymous cryptocurrencys will not overtake Bitcoin for the simple fact they are more anonymous, it will take much more than that. I think it is possible that another cryptocurrency eventually surpasses Bitcoin, but Monero is not that cryptocurrency. Simply making transactions more anonymous is not a big enough innovation to overcome the infrastructure and network effect of a crypto currency like Bitcoin. It will require better anonymity couple with many other features and/or improvements to do this.
There are private transactions, anonymous transactions and plausibly deniable transactions - each coming with tradeoffs. I could see situations where legit and illegit companies/persons would want to conduct private transactions but not necessarily anonymous nor impossible to prove transactions.
Organized criminal enterprises, high net worth individuals, intelligence assets (spies), etc all have reasons to want to prevent transactions from being discernible to outsiders but might want to be able to prove they made or received payments to certain 3rd parties. Even mobsters have accountants.
It's reasonable the feature set Monero offers could be superior to the one proposed by Zerocoin, under a broader set of circumstances.
Ask yourself this. Would someone looking to replace a swiss numbered bank account chosen for tax avoidance likely use a tool that will only be perceived useful to criminals or one that has broader purposes? I posit that Monero would be more palatable.