Will BitCoin be first adopted by the underground, black markets, criminals, money launderers, etc.?
We can safely assume that if criminal markets are aware of Bitcoin, then they are already using it. We will never really know for certain, except for those of us who are directly participating; which would be the attraction.
The one thing I'm most worried about is that someone (or some company) will develop some kind of proprietary hashing hardware and monopolize all new currency creation (like DeBeers with diamonds). This might be solved by a break in the chain to change the hash function to something more resistant to specialized hardware.
This is not something to be worried about. In fact, something along these lines is a near certainty, but a monopoly is impossible. It is generally predicted that, in a future that Bitcoin is at least as much of the economy as PayPal represents today, most generation will be performed by major financiers who have an unusually large vested interest in the security of the blockchain. They will be defending their own balances from the other
major players. These major players are likely to function like banking institutions today in many respects, as the average Joe isn't likely to run his own client, but instead have an account at a particular major player's website, similar to how Mybitcoin.com functions today. Regardless of how the future looks, Joe would still have the option
of running his own client, including generation, for whatever reason. Perhaps he has lost trust in his 'bank', or simply desires to heat his apartment with the off chance of catching a block. His reasons do not matter, only that he continue to have the option.