You can get it either by getting Dark Elements, get it by getting a Random Dragon Egg, or you could get it when you "discover" 33 fully grown dragons in your dragon book. If you read the description with the dark shadow of the dragon, it says, "A very rare dragon which cannot be obtained during adventure" Or something like that
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Well there should be one that is green and black ichose it and got a really strong dragon you need a dragon amulet to hatch it!
You can't.
a green dragon will still come out no matter what is the colour of the egg.
kill the ender dragon
Your first dragon has to be at least level 30 before you can claim another egg