Answer:
16 marbles.
Step-by-step explanation:
Since your friend is blindfolded the only actual way for him to be completely sure that they picked out all 6 black marbles would be to pick out all of the marbles until none are left. As long as there is even one marble in the bag then your friend will still not be sure which color it is due to being blindfolded. Therefore, under these circumstances the answer would be 16 marbles.
