Sometimes in the open game tab, I wonder, why some people are marked as blacklisted, ofc because I put them there, but I have to check previous played games to get to know the reason.

For example, if there's a 6FFA with one seat open, I wouldn't want to play with a, let's say, "rude person", so I have to check previous games. If, however, he's simply there because he quit my team in a teamgame at turn 1, or he played even worse than an AI, I wouldn't mind playing with him in FFA.

You should be able to select an option for why you intend to blacklist them, so if you mouse over the small B, instead of showing "---- (blacklisted by you)" there would be "---- (blacklisted for being/doing/having etc.)