The more minerals the harder it is to work. So jet black could be that. But then there are also colors like cadmium based that too much heat will cause to boil. Cadmiums we feel are the hardest to work as they take the most time. No color is really harder to work than another, other than the time it takes.
Adding dichor or opal can also make the color harder to work per say. It does not really make it harder to work either, but if you work it too hot, the dichro or opal will burn out.