I think I wrote a similar blog a long time ago and I'm sure others have written similar ones too, but I feel compelled to go there again. The reason I Blacklist is simple: Life is too damn short to let people irk you.
I blog for entertainment, social interaction and to keep my brain from total atrophy. I have enough stress in my life that I don't need strangers on the internet to supply me with more. I am human so sometimes, depending on my mood, it doesn't take much to irk me. The fact of the matter is, you don't have control over what irks you sometimes but you can have control over whether or not you allow it to continue. That's where Blacklist comes in handy.
I hadn't blacklisted anyone in a long time. I recently did and the person responded with a "poor me" blog that I will respectfully stay clear of. I just want to say that I'm aware other people have problems. I'm just not interested in them demonstrating them on my blog. Life is too short to put up with people who haven't earned the right to irk you (i.e. people who know you and have made themselves known to you over time) giving you any kind of grief.