On the topographic map of the Plitvice Lakes National Park in the area of Čorkova uvala, there is a locality called Crno jezero. In legends, it is also connected with The Black Queen and the formation of Plitvice Lakes. As this is a forest area with only a few smaller springs and occasional watercourses, it is clear that there is no talk of "ordinary" the lake.

Only the older inhabitants of this area, long-term employees of the Park and old records about Plitvice Lakes witness the existence of a lake called Black Lake.
People's voices about the underground venting and the distant reaching of water from the Plitvice Lakes
D. Franić, Plitvice Lakes and their surroundings, 1910.
Water flows into the Plitvice Lakes, as already mentioned, visibly and invisibly, and also drains out. There are several springs on the side of the lake, which flow into them as well as by the Plitvice stream, but these springs are lost in the summer, so the "Black Lake" from Čorkova Uvala flowed (underground) into the Kozjak lake.
It is an occasional sinkhole into which water infiltrates from a nearby permanent but very small karst spring called Antino vrelce. The tracings, which were carried out in 1984 by Dešković and colleagues, proved the direct connection of the Black Lake sinkhole (829 meters above sea level) with the source of the Plitvice stream (610 meters above sea level).
///Did you know that there is also a rainforest in Plitvice?

Yet few had the opportunity to see it as a lake. Only in short periods of extremely high water, after the sudden melting of large amounts of snow or heavy rainfall, does the sinkhole fill with water and become a lake.
This sinkhole, unlike others, fills with water due to the partially waterproof deposits at its bottom. The water stays in the sinkhole for a short time, depending on the hydrological conditions, and usually after a few days of sinking the sinkhole regains its previous, usual appearance.
Black Lake and Black Queen
On the 70th anniversary of the Plitvice Lakes, in May 2019, due to extreme hydrological conditions, we had a very rare opportunity to see Black Lake in all its glory.

///Legends about the origin of the Plitvice Lakes
The Black Queen said to them: “This is your land and your homeland, the only one you have. And this rainless and hard, ugly and barren land is the most beautiful and richest land for you. Love her! I am your daughter and I am not a miracle from heaven. I am your love and hope. I heard you and recognized your hearts. You have freed yourself from the evil of humans from your tribe. You who spoke - spoke the truth. That is also my truth. Disperse to your homes, and the rain will come again on the thirsty fields and forests, and you will again have the most beautiful land for you. And others will come to admire her. My tears will create water: the right eye for the black spring of the Black River, and from the left a white spring for the water of the White River."
///Barbara Celjska – Black Queen, Mistress of Medvedgrad
In that, the rainbow disappeared, and two clouds appeared on the horizon above Vrelo: one dark, the other white. The first drops of rain kissed the dry ground. Everyone stood with their hands outstretched to the sky, as if enchanted, listening the birth of the source of the Black River. Since then, the water has been flowing continuously and non-stop, bubbling in a series of sixteen beautiful lakes.
Source: National park Plitvice Lakes