Why do copper vessels bottles or any copper utensils turns black from the inside?

Usually, copper vessels turn black from the inside after regular use because it forms copper oxide by reacting with oxygen in water. 

When you store water in a copper bottle overnight (8-10 hours maximum suggested), copper ions dissolve in that water in small amounts. This process is called the 'Oligodynamic Effect' as this water now has the potency to kill harmful microbes, fungi, and bacteria.

If your copper vessel has blackened from the inside, there are a few steps you can take to restore it. One common method is to mix equal parts salt and flour with enough vinegar to form a paste. Apply the paste to the inside of the vessel and let it sit for a few hours. Then, rinse the vessel thoroughly with water and dry it with a soft cloth. This should remove the blackening and restore the shine of the copper.

Alternatively, you can use lemon juice and baking soda to clean the inside of the copper vessel. Mix the two ingredients to form a paste, apply it to the inside of the vessel, and let it sit for a few hours. Then, rinse the vessel thoroughly with water and dry it with a soft cloth.

It's important to note that copper vessels should not be cleaned with abrasive materials, as this can damage the surface of the copper. Additionally, it's best to avoid storing acidic liquids in copper vessels for extended periods of time, as this can cause the copper to corrode and blacken.