NOTES
It's easy to see that the final percentage will be very close to 50%; here's the result of a simulation of the process - note that the percentage of black marbles jumps around a lot in the early stages, before settling down to 50%. Now imagine reversing the rules - if you pull out a black marble, replace the black marble and add another black one; if you pull out a white marble, replace it and add another white one.
Now what do you expect?