They are all faded into grey, no real black in this shot. I like the idea of desaturating the one olive. Although the pimento in it gives away the fact that it was green before desaturation.
I would've boosted the contrasts on this much more, as it stands now your Black olives look grey. I like the idea you were going with, using the lighter olive to offset the black ones.
Where is the focus in your shot? The black olives appear to be grey and the green one looks light grey. I think b&w didn't help this shot but I do know that focus would have. Good luck in the challenge
You should have took the black olibes in oil... You would have get a deep and shiny black. In converting your picture in gray scales like you did you lost some of the richness of the tones... My opinion!!
Composition is good - love the placement of the green olive, and it fits the challenge well, but the focus needs to be sharper to really make this "pop".