Jack Handy would consider speaking negatively about another person as falling off of the positive thinking wagon. Yet venting is a powerful tool in resolving emotional upsets. Venting done right leaves us feeling better afterwards, we become less stuck within ourselves. This is not to say that speaking negatively of others is without fault. Venting done wrong is poisoning and/or blaming.
Poisoning seeks to enroll others in the permanency of the negative circumstance, and the ‘negative circumstance’ may be another person. Not that the other person did something bad, but the other person is bad. That the problem is outside of ourselves.
Blaming and poisoning may be synonymous, but I might add a couple of distinctions: first, and most obviously, you may blame the person to their face, while poisoning is done to someone else. Also, blaming in a relationship expresses a debilitating perspective: this problem is outside of myself, there is nothing I can do about it.
A poison conversation is one in which negativity is expressed with the intention of convincing. My goal becomes to make your position my position. The poison mindset is emotionally invested in the continuation of the problem; it enjoys the existence of the problem; it wallows in the negativity. It imagines worst case scenarios which pollute interactions: ‘Jim is late to our meeting. He must be plotting to screw me.’ How will you be received by Jim next time you talk to him? Will that incline him to being sympathetic to your upset?
Poisoning and blaming are not constructive activities. They are debilitative mind sets, best replaced with more facilitative perspectives. More facilitative perspectives typically revolve around personal inquiry: what is my contribution to this matter? what can I do differently to get better outcomes next time?