I've googled it and some people say it does, while others say it doesn't, does anyone have an offical statement about that or tested this on his own?
According to the majority of the posts of the D3 forums, it DOES count.
This is the best explanation I found: Knockback tends to refer to any effect that moves the target, usually making them airborne. The direction of the movement tends to be irrelevant for it to count as "knockback".
A very good thread about Black Hole and knockback can be found here: http://us.battle.net/d3/en/forum/topic/12373517571