Is it OK to contact someone who is on my suggested reviewers list to discuss something unrelated? I submitted a manuscript to a journal. As it was required, I included a list of suggested reviewers. Of course, I don't know who of them have been selected by the editor. I haven't received any reviews yet. Several weeks have passed since the submission. 

The problem is that I forgot whom I suggested as a reviewer. These were some people from my field and I have alerts on Google Scholar that notify me about their new papers. Recently, I got such an alert about someone's new paper. I found it interesting (but not really related to the topic of my submitted manuscript) so I wrote an email to the author. (I only met him once at a conference.) I thought that he found something similar to what I saw in a paper of yet another author. We exchanged a few emails. The discussion was totally unrelated to my manuscript.

Only after that discussion I realized he might have been a reviewer of my submitted manuscript. So I wonder whether it is OK to contact someone who may  be my reviewer? The review process is blind so I never know for sure.
The world of academia is small enough so that you'll likely get to know most (or all!) reviewers for your work. You can't be expected to stop communicating with your colleagues, just because one of them could be reviewing one of your papers right now.

So yes, go and discuss.