I can't speak on the black market number because most measurements of the black market are very difficult to verify. However we can measure worldwide GDP and the supply and demand for currencies presently, and can represent them in USD.

If all 21 million Bitcoins were in circulation (which they are not, and over a million may simply be "lost") today, and only 1% of the World's GDP transferred through Bitcoin, the value of a single Bitcoin would be ~$2200/coin. These are using all the most conservative estimates, because we're obviously not at 21 million Bitcoin, and we're probably still a long way from 1% World GDP, but I do think that target is attainable in the next year, and, given the success of Silk Road, could be attainable within one year, we just can't measure it as legitimate world trade.