TheLorax, I think you're complicating things too much. What you say about the chances changing depending on how many marbles are left is true. And of course once all the black marbles are gone there is zero chance of getting a black marble.
Try another scenario. Suppose every one of the 30 employees put their hand into the bag exactly at the same time, and pull out a marble. Each employee has a 1 in 6 chance of getting a black marble. The is the model which explains how each employee's chance of getting a black marble is 1 in 6.
