Is it a BLACK-HOLE?
A Vorex of some kind?
No - not a Black-Hole - it is the inside of the flue pipe of a stove that has been burning unseasoned wood.
Do NOT buy unseasoned wood
As you can clearly see in the above photo, about two thirds of the flue-pipe is blocked with this creosote encrustation. If much more of the pipe was to become blocked, there would be a very real danger of CARBON MONOXIDE being released into the house.
Carbon-monoxide is a KILLER.
Here is another example of a creosote clogged flue pipe. The above example is getting very close to the point where carbon monoxide production would become a real possibility. If you burn a lot of wood - please check your flue pipes regularly.
Another real danger with Creosote
There is another danger involving a creosote build-up in a flue. Creosote is flammable, if the build-up in the flue-pipe ignited, it would go off like a rocket. It could pose a very real danger to the house.
A metal pipe will actually redden and you would have a roaring noise like a rocket. It will cause expensive damage, and may well set your house on fire.
Yet another but lesser problem is the difficult and dirty job of cleaning out the flue pipe, and if you use a lot of unseasoned wood, that should be done at least twice in a season. That hard work alone makes it worthwhile to avoid using unseasoned wood.
If you burn kiln-dried or fully seasoned wood, there is a greatly reduced amount of creosote released. With wood briquettes with their very low moisture levels, creosote is hardly a problem at all.
Please be warned - do not buy those wood blocks in plastic bags unless you 100% trust the person who is selling them. Most of that stuff is green wood, not properly seasoned, and I would bet a weeks wages, if I were a betting man, the chances of that stuff being kiln dried is about 100 to 1 against!!