"Quantum Dot", as implemented, is the same thing as "QLED". It is a type of backlight used for LCD panels (TN, VA, or IPS). Typically it uses red and green quantum dots combined with a blue LED backlight, sometimes called "QLED". Quantum dot backlights may be used with TN, VA, or IPS panels, it is an entirely separate component from the LCD panel and is not better or worse than any panel technology. It is better than "LED" backlights previously used with TN, VA, or IPS panels.
Some people (articles) have used the term "QLED" to describe the concept of a display that uses quantum dots as pixels directly, but there are no commercial displays of this type in existence, and so far the usage of the term "QLED" by the industry has been to indicate quantum dot + LED backlights for LCD panels.
@OP for the display to look like it's off when showing a black screen, you need an OLED display. There aren't any OLED monitors yet, aside from a Dell model (U3017Q) that was discontinued a while ago.