If you are shooting in HD, the video is widescreen by default. If you want to add black bars to that HD video, download
plugin and apply it to every clip you want the black bars to be (or to an adjustment layer on top of everything if you want the same bars to all your footage, but if you apply to every clip, you can reframe every clip up and down if they need to).
If your final delivery is SD (which I think it won't be) just edit everything in an HD project, when you finish you can export a master file for yourself and then import it back to FCP X in a SD timeline and the black bars will be added automatically.
Hope it helps! Let me know if you have any other doubts.
Just desaturating in the Color Board wont give you any good looking B&W. There are many B&W filters for FCPX that will do this better, if you do a google search you should be able to find one that looks interesting for you.
This said you don't really need to pay for any filters to do this. There's an old film processing trick for turning color video into punchy B&W that you can also use in FCPX. I have made a little video about this, if you want you can download it here: