I'm not much of an antagonist but anyone who is saying black pastel is wrong. They look nothing like the morph black pastel. They are too bright. Also the people saying that they are black back pastels I feel are missing something. The sire is both BLACK BACK and PASTEL looking. That's 2 different morphs. Perhaps the OP stumbled upon a "morph" that has both built into one snake. If it was indeed a 2 morph animal, the odds gods were very nice to him. They way I see it, if it is a black back pastel (2 morphs) there would be some normal black backs and some pastels in the clutch.
I think the clutch is awesome and to help you prove them to be something, get a black pastel and a pastel and breed the same "new morph" snake to both. That clutch should be fairly convincing on what this "morph" is.
Also how morphs happens is RANDOM CHANCE. The sire is very subtle if you don't know ball pythons. Perhaps the person who hatched him out in the first place thought he was a normal. Then the OP proves there is something going on. Technically you could breed 2 normals and get a banana. Very unlikely but possible.
1) Don't think anybody said it looked like a black pastel.
2) How does breeding 2 normals produce a dominate morph? I mean, sure you could pop out another mutation but what are the odds of the same morph randomly mutating twice?