Prior to a black hole reaching sufficient (size ) to stop radiating light (due to excessive gravity ) would the speed of radiation of light slow smoothly down to zero (as the hole draws closer to the critical size)
The local speed of light, as measured in the coordinates of any inertial observer, is always the same. There is no change during the formation of a black hole.
The coordinate velocity of the speed of light could change during formation of a black hole, depending on what coordinates you're using. This is of little interest, because you can always make a coordinate velocity be whatever you like, by choosing the necessary coordinates. This is true regardless of whether there is a black hole or a gravitational field.