Is it legal and safe to take off or land in zero visibility?
In the US, for general aviation flights, operating under Part 91 of the FARs, it is legal to take off with zero visibility (see page 2-2 of the IPH, PDF). In my opinion, it is not safe. If an emergency were to take place, you would be unable to return and land; under Part 91, it is not legal to land unless conditions are equal to or better than a runway's published approach minimums.

Unless you've got a great alternate nearby, conditions are improving rapidly, or you've got an extra engine or two, you probably shouldn't take off in conditions you couldn't land in.