First the ultramel. There was a new hypo gene discovered that was allelic to amel. Meaning they both had the same locus. So if you breed an animal that is homo ultra to one that is homo amel, they will each donate one copy of their respective gene and you will get ultramel; which will be an "in-between" between an ultra and hypo. There will and can still be some black, though greatly reduced. Amel eliminates it, ultra reduces it - so it will still be there. Though this can vary from animal to animal. Some will have virtually no black and look closer to amels; some will have a fair amount of black and look closer to hypos (ultras).

Now, lets say you breed an ultramel to a normal. The offspring will all be normal het for either amel OR ultra. This is because at that locus, the parent can donate one copy of the gene. In this case it has one amel copy and one ultra copy - so it will give either, but not both. This is why you can never have an animal that is het for ultramel. If you were to breed two ultramels together, you would get a mix of ultramels, amels, and ultras.

It's a little confusing to grasp at first, but hopefully this makes sense.

Now, about your hybrid question. If you knowingly breed a snake that is not 100% pure corn, then you should label the offspring as hybrids as well. In your case since one is 25% king, then the offspring will be 12.5% king and are still hybrids. I would consider it to be dishonest if you did not disclose this info to a buyer - regardless of how "pure" they looked visually. So even though you're diluting the hybrid part, it's still a hybrid and should be labeled as such.