"I had to blow off steam" would be appropriate if your main emotion was anger or outrage. "Thanks for letting me vent" would do for whatever strong emotion you were expressing (as would a simple "thanks for listening"). You could also refer to it as an "outburst" (but not an outpouring). Your idea of following with "I had to share my feelings with someone . . . " is a good one.
There is still no contextmeaning that we do not know what sort of emotion we are talking about or who you are talking to.
Forget about 'outpouring'. I think 'outburst' is an excellent word, generally understood and covering the most likely emotions of most situations.
'Venting' is a useful word, but I do not know how common it is. I suspect that older
(50+ ?) British/ European people, who do not use the internet for general communication with a wide variety of people, might not be familiar with it, but I really don't know. To me it is a modern word that I encountered first on -line and I've never heard a British English speaker use it without an air of self consciousness. My comment is based only on my own limited experience.
"Letting off steam" means much the same, especially if emotions of anger and frustration are involved.
Another phrase, very informal, that might be out of fashion these days, is "letting it all hang out". This might be used when more personal emotional issues have been involved in the 'outburst'.
'Unburden oneself' is also a possibility. "I'm sorry for unburdening myself the other day". It's probably more formal and serious.