To start with, the main reason no one working on the LHC is too concerned about black holes is because of Hawking radiation. While we usually think of black holes as objects that nothing can escape from, Stephen Hawking predicted that black holes actually do emit some light, losing energy (and mass) in the process. In the case of the little bitty black holes that the LHC could produce, they should just evaporate in a shower of Hawking radiation.
That's great you say, but Hawking radiation has never actually been observed. What if Hawking is wrong and the black holes won't evaporate? Well, the usual next argument is that cosmic rays from space bombard the earth all the time, producing collisions many times more energetic than what we'll be able to produce at the LHC. To me, this is a fairly convincing argument. However, let's pretend we don't know about these cosmic rays and that there's no Hawking radiation. We can calculate what effect black holes produced by the LHC would have on the earth if they do stick around.
