I bred my blue dragon yesterday with a sky-blue mate in someone else's lair.

Both dragons are wild-born, according to the pedigree.

Today I got a hatchling. And the hatchling is ... black! (or Silver* as it says on it's page)

Is it normal? Is it a mutation color from a recessive gene that's not listed?

I found it strange that the baby looks like none of the parents. I like surprizes, don't get me wrong, but how do genetics work exactly? :/

First thing, the parents could had silver recessive.  Dominats are green and grey means its a recessive gene.

Next you got lucky and got a deviant. 

Deviants, are complicated to explain.  Deviants are unusual colorations of 3 pets on, unicorn, dragon, and roc.  The concept,  basically you breed to animals together that have same pattern or same color  (pattern and color is a lot cooler).  Then you have a 10% chance of getting a deviant. If I am correct if you have color and pattern you have a 25% chance to get a deviant color or pattern, and a 10% chance to get both.  Here is a list of skins that might make visuals a little bit easier.  List of Skins.

Deviant Pattern:

Mother has stripe

                              Baby 10% chance deviantion of stripe

Father has stripe

 Deviant Color:

Mother has Buckskin

                               Baby has 10% chance deviation of buckskin

Father has Buckskin

Double Deviant:

Mother has stripe & buckskin

                               Baby has 25% chance deviation buckskin or stripe, 10% chance both

Father has stripe & buckskin

Bet your mind was blown right there, boom.  If it makes no sense, dont worry it will I promise.

Wow, thanks. o.o

That explains it. Well, at the moment she looks just like a simple black dragon, altho, it says it's 'silver'.

http://beastkeeper.com/pet/450048

Will it morph into a different color as she grows up?

I am new and haven't really seen too many hatchlings to tell. I'd really love to get one of those lovely rare colors.

If you look at the list of skins located under "help".  There is a couple different types of black to, striped, diluted and others.  But once it is an adult it will either have an odd color or an ood pattern or BOTH! You can look at the skins and predict.

Hello! I had the same thing happen to me, I took one of my bought rocs and a pound roc, bred them together and wala! A lavenar deviant. It was very surprising. I guess you and me were very very lucky. Theres also an explanation to deviant colors and patterns FAQ... if you want to go and check that out.

So... Just clarifying some stuff

To get a double deviant, you need to breed a pet with deviant color to a pet with deviant pattern, then the baby has a 1/100 chance of being double deviant. 

If you breed normal pets, then the chance of deviance on either color or pattern is always a flat 10% when the baby has two of the same gene (for color and/or pattern). If RNG hits the 10% chance of deviance, then it chooses one of the homozygous genes (Color and/or pattern) to make deviant.

See this thread for details and examples: http://beastkeeper.com/thread/6137

Your pet hit the black dilute color - The name is silver, but this version of silver uses the black gene, which is why its showing up black as a baby. 

Both the mother and father have dilute, so I'm guessing the baby is dilute/dilute, deviant dilute. However, you don't know what the mother's recessive color gene is yet, so it could also be Black/Black, Dilute/Spotted with deviant black. Or Black/Black, Dilute/Dilute, with either color or pattern deviant... but not both.

Wow, this game is exciting! ^0^

This is another baby from one of my gryphons, out of a fling. It seems to have piebald. My male gryphon has piebald recessive. :)

http://beastkeeper.com/pet/450424