The skin, meat, bones, and major organs are generally "black." Silkies are common at Asian markets here -- heck, I bought one this past weekend (for soup!) for $7.99, and there were literally hundreds available.

I wish we had them when I was a kid; they look pretty funky.

The Wikipedia article makes it sound as if they all have black meat (not just the black ones.) I guess that would be a good reason to not eat them (convincing children they taste good might not be worth the effort if you have regular chicken anyway).

We tried to keep some because they have a good disposition and because they're different, but we also had some Reds and the Reds just tortured them. It was ridiculous. The meat is blue/black.

A lot of farmers keep silkies because they make excellent surrogate mothers. You probably wouldn't eat them very often if you were using them as such.