Personally, I would find this INCREDIBLY useful. I'd like the ability to leave a small blurb such as "Bad trader", "scammer", "social menace", "bigot" or whatever along with blocking someone, that would show up privately and on the list next to the blocked person. I frequently block people for short periods of time, and remove them once I've forgotten why I'd blocked them....but honestly....it's kind of gotten to the point where I just don't even see the point in interacting with certain people ever again. I'd like to be able to separate out the wheat from the chaff so to speak, and currently, the only option I have is to either not block someone, or accept that at some point I won't remember why they're blocked and leave them permanently blocked.
Otherwise, I guess I need a spreadsheet for this crap now. =|
Even just the ability to go in and annotate these from the block list would be extremely helpful if integrating the dialogue alongside the block button itself is a pain in the butt.