You don't get a chick that is "blue and splash."
Splash is a color that includes bits of blue. (That's why it's called splash: there are splashes of blue on the chick.)
If you are talking about hatching chicks, having some blue ones and some splash ones but no other colors, then cross a blue chicken to a splash chicken and you'll get about half each way.
Mottling (white dots on a black chicken) is caused by a different gene. Not the one that causes blue and splash.
A chicken can be black mottled, blue mottled, splash mottled, or something-else-mottled.
Mottling is simply recessive. A chicken with only one copy of the gene does not look mottled at all. But a chicken with two copies of the gene (one from the mother, one from the father) does look mottled. So if your rooster has one copy of the mottling gene, and at least one hen has one copy of the mottling gene, then you can get some chicks that are mottled.
Have any of the chicks grown up yet?
Black chickens often hatch out looking like black-and-yellow penguins. And when they grow their first sets of feathers, it's common to have a few white feathers. And then they grow up to have solid black feathers.
(Chicken colors are weird things!)