Did you guys use IVF there could have been a mix up but some of the others could also be right one or both of you could have had a dark complexion

The real question here is did SHE even know prior?

You should really do a DNA test first. Genetics is a very complicated thing, and it’s not very likely two White people can have a dark skinned child, but it IS a possibility. Especially if you two aren’t very knowledgeable about deep roots of your family tree (not blaming you or your wife). You can wait to sign the birth certificate, and you only need a little blood for a very thorough test. It’s important to know right away, instead of throwing away your possible child and your marriage because of an uncertainty. It’s better to be 100% sure, than 95% or 90% sure.

Just because your kid is black doesn't mean your wife cheated on you. Were either of the kids grandparents black?

Get a lawsuit for emotional distress

DNA test. If it confirms your suspicion dump her and try to make sure she gets as little as possible from you and cut her out of your life for good. Sorry that happened, can’t even imagine how angry I’d be.

Why the heck is this whole idea of the kid winning the genetic lottery getting liked so much? News flash, the kid isn't yours. Let your future ex-wife and the biological father pay for it. Don't be a sucker. Take her to court, get a proper paternity test done. No lawyer in their right mind would try to argue some genetic lottery win in court when they know a DNA test would easily prove otherwise.

"Honey the baby's not black, he's just tanned"!

If you both have a recessive allele for 'dark skin' which was passed on to 'your kid ' it is hypothetically possible but highly unlikely

did they change the baby at the hospital?