What is slump glass?

Slumping glass is a highly technical operation that is subject to many variations, both controlled and uncontrolled. When an item is being slumped in a kiln, the mold over which it is being formed (which can be made of either ceramic, sand or metal) must be coated with a release agent that will stop the molten glass from sticking to the mold. Such release agents, a typical one being boron nitride, give off toxic fumes when they are first heated and must be used in a ventilated area. The glass is cut to the shape of the mold (but slightly larger to allow for shrinkage) and placed on top of it, before the kiln is heated.

What is the most notoriously hard borosilicate color to work with?

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.