The keys doesn't sound plain different, they are slight variations.
Generally, keys on different rows make slightly different sounds. The sound of the switch is strongly amplified by the keycap, without a cap it's barely audible. Different rows have differently shaped keys, giving the klick a higher or lower pitch.
Within rows, sounds differ a little too. I suppose that's due to different positions on the PCB and in the case, which acts as a resonance chamber to some extent.
The click overall is very quiet, sometimes hard to hear.
This is a bit puzzling to me. Usually new Cherry boards with blues are pretty loud. But I doubt all switches on a new board are broken.
I have no experience with black keys though. Are they generally more quiet, maybe made of softer plastic?