stain much darker than the rest...

sometimes, seriously awesomely good stain just does that!  It always happens to my palms and knuckles.  There's nothing to worry about, it just means you have really good powder and you stained really good. :)

06, huh?  Good stuff, still!!!

Could be you have calloused skin there.  hyper receptive to lawsone thus hyper dark long lasting stain. 

Its not unusual to have one single spot that gets crazy dark.

Henna will stain callouses and skin with papilloma virus differently than the rest of your skin.  Have you ever had warts?

Even if you had a wart when you were ten, the structure of your skin is still changed.  Skin that's been infected with papilloma virus is highly keratinized (even decades later, it'll regrow differently from adjacent skin) .... and going green/black is apt to happen. That's probably what you're seeing!

I've seen this before a couple of times.  Indigo also stains differently on skin that's had structural alteration from damage ... alterations that you'd never see just to look at the skin.

Could just be that there are more layers of skin to stain there. 

I wouldn't worry about it. Sometimes I get a stain that dark on the thicker parts of my hand. It just means you got really GOOD dye release, and it's not a problem at all.