Hi Evolving, if I say something you already know, please excuse. I will try to be thorough in case someone else sees this and needs the extra info.
Each kittycats has a shown and a hidden for each of their traits. When they breed, each parent contributes either their shown or hidden of each trait, one from each parent which forms the new baby's shown and hidden trait. The more dominant of the 2 will be the shown and the more recessive will be the hidden. In some cases, the shown and hidden trait are the same which is referred to as a pure trait.
Your mom cat shows blue marble and either hides something more recessive or is pure. Your dad cat shows the seal and hides either blue marble from his mom or her hidden trait (which could also be blue marble). You would think that the more dominant fur would show up more often in babies but as you can see, that is not always the case. It is entirely random but you can increase your odds. I will explain:
You would have a higher chance of a seal baby if you mated your seal to another seal even though neither is a pure seal.
Let's call Seal 'S' and any recessive fur 'r'. The traits for the parents in your picture would then be Sr X rr (Seal hiding something crossed with something hiding something, as long as 'something' is more recessive than seal, its fur doesn't matter) so the resulting baby fur possibilities would be Sr, rr, Sr, rr. That is a 2 out of 4 or 50% chance of a seal baby with each random box.
If you breed 2 non-pure seals together, the parentage would be Sr X Sr and the box fur possibilities for those would be SS, Sr, Sr, rr which is a 3 out of 4 or 75% chance of a seal baby with each random box.
To get the siamese seal 100% of the time, I can think of 2 options:
1) Breed 2 pure siamese seals together, this would be SS X SS so there is no question, the baby would also be pure SS
2) Breed 1 pure seal with any cat with a less recessive fur which would be SS x rr with 100% chance of a Sr offspring (Seal hiding something else)
I hope that clears up some things or did I make it more confusing?