Blob hatched from eggs is predetermined on the day it was laid. With that information, it provides you with two options:

1. For Gold Making
Save manually before hatching. Hatch and see whether you like what you see. If you don't like it, reload the saved file and choose to sell the egg instead.

Keep in mind that you CANNOT sell a blob. You can sell the egg, which goes for 400.

2. Getting the Right One
So, you want a particular trait in the blob? Sure.

Save manually the day BEFORE the egg is laid (you can see the progress on top of the mating blob where it should be at the 3rd green bar). Sleep and hatch the egg. Don't like what you see? Reload the manually saved file, sleep. Rinse and repeat until you get what you like.

Things which you can reroll: Skills, Mutations, Looks, Ability, Element type
Things which you CANNOT reroll: Power

I hope this help