Have you ever come across a "Black Hole"?
The person who absorbs all the energy in their surrounding environments.
How would you describe a black hole personality?
What type do you think the black hole person would be?
I don't think I'd say that it's necessarily something that's driven by type much. Much more so on attitude and openness (which I'll define here to mean receptivity to the states of people around them). People who are down, but who can take a spark from others and lighten up aren't black holes. People who radiate doom and gloom irrespective of their company and said company's efforts to brighten things up can pretty much kill off any situation. People who consistently demand to be the center of attention can be black holes too.
I guess the only thing I'd say in how such things might relate to type would be the mechanism that the black hole uses to kill fun. Different fun-sucking, hope-killing strokes for different folks, I guess (score one for optimism!

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Yeah, I agree. An unhealthy ISFJ in a bad mood can suck the energy out of the happiest person on earth.
I don't think that this is necessarily specific to ISFJ's, but from personal experience... hell yes. Like in a "you walk in the room and instantly start to cringe" way. Perhaps I'm just more vulnerable to the vibe unhappy ISFJ's give off, but yikes (happy ones, of course, I really enjoy being around).
A good way to answer this question would be for people to think about ways they have or could be a black hole.
Good observation, PM

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I think (unfortunately) that I can definitely be a black hole at times. Granted, one with a small event horizon (because, as Jock mentioned, I'm far more likely to be unnoticed than not). Two situations offhand that I have/could be a black hole (that I know of - feel free to add more

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1. Social Settings: - usually I blend into the background, but if I'm in an engaged, lively environment and called out on to "join in" - my awkward refusal/demurral can definitely kill the mood.
2. Professional Settings: - I can become the ultimate nitpicker and naysayer at times - I just don't let things go. If someone is glossing over an idea ("Oh, yeah, we haven't addressed that, but it'll 'work out somehow'"), I'm the one who says stuff like "Are you sure? What about X?". While I think I'm being constructive, it often kills the mood and sucks the life out of the meeting.