Well, it's not a darkness chemical but you got the idea of it.

The plant measures the uninteruppted darkness, and when you swtich to 12/12 it'll measure the uninteruppted 12 hours and release the flowering hormones.

Now i've had a better look i agree with Markass, that main stem is pretty damn small - i'm guessing you didnt have a fan? Well, if your going to flower it now, and if it turns out to be female, you'll wanna get a fan on it and you'll probably have to tie it up so the weigh of the buds dont bend it right over. But still, it's fairly healthy and even though it might be a small harvest it'll be smokeable.