I would like to create a transparent black rectangle but I don't want the colour to turn grey when I turn down the opacity (around 60%). I want to retain the black colour of the original shape. How do I do that?
In the example below you can see how opacity works.
We have four squares in different colors. The outer colors are without any alteration: white, red, green, black (clockwise starting left top).
The first overlay is white in 50% opacity. As we can see it will make colors lighter. Except for white, because this already is the lightest and can't be any more white.
Similarly we see that a black overlay will not make black any darker, but will darken other colors.
So you cannot make black transparent and still retain black, unless you put it on a black background (but then there's no point in the transparency).
Copy those blackstaying ojects into different bottom layer which is kept 100% opaque.
For a color physics aware person it's easiest to think the asker to be a lunatic. But you aren't! I think you want to see non-black objects through a black object just like you can see a light from the bottom of a deep well. You may consider the top opening of the well as a black object which is transparent.
In physical world the top opening is nothing, it is totally empty area and has no color - not black or others. It's only an illusion to see a black transparent object on the ground.
Altough geometrically in lower altitude, than the surface of the ground, the light at the bottom is still on the top of existent materials(excluding the air which is transparent)
As stated above if you lower the transparency of a black object it will appear lighter in shade (unless it's against a black background). I think you can get a black shape that reveals images under it with the transparency tools. No, I don't know which one, I play with all of them to achieve cool results. Place your black rectangle over other art you want to reveal, then apply transparency settings to the rectangle.