Sure there might be a decline in the price of bitcoin, but I don't think bitcoin will fall that much. When the silk road was shut down, more sites popped up and when that site gets shut down, more will pop up. So the people who are buying off those websites would just wait a while and then find a new site and resume buying.

Unless the entire dark net would be taken down, which seems impossible I don't think bitcoin won't fall that much.

As a matter a fact, if the dark net was shut down, bitcoin might actually go up, since a lot of people still think bitcoin is built off the dark net. People would have a much more positive view on bitcoin then.