Blue is an incomplete dominant gene to be blue your bird must have 1 blue gene and splash is when they have 2, lets say this is the blue gene B and this is black b, your blue is Bb and the black is bb
Top row is the blue side is the black
B b
b Bb bb
b Bb bb
You can see from this cross you have a 50 percentage chance of either colour per chick
Splash crossed with black.
Splash BB, black bb
Splash top, black side
B B
b Bb Bb
b Bb Bb
With this cross you have 100% blue
The percentage of what colour you get is per chick, so if you have 50% chance of getting a colour and you have 6 chick the batch won’t be half one colour half the other, each chick will have 50% chance of being either colour
I am not sure what you mean with the mottled, mottled is a pattern which goes on to other colours, so you can have black mottled, blue mottled and splash mottled (I don’t know much about mottled)