Blue Black and Splash are a case of incomplete dominance.
Here is one. On the top is one parent a Black(bl+ and bl+). Not sure what the plus means, incomplete dominance perhaps.
On the left is the other parent, who is a blue(Bl and bl+). Bl is capital because it is dominant over the black. So even though the Blue parent caries the bl+ gene, it is blue.
When making sperm(or egg) one sperm gets one gene and another sperm gets the other gene. 50% of the sperms will have either of the two. Same with eggs.
When that mixes with the other egg or sperm, you get new combinations. If the offspring carries the Bl with a bl+, it will be blue. If it only has Bl+, it can only be black. It did not inherit the splash gene from the blue parent.