I would never advise taking Black Hole when you could take something useful like Hasten, a Leadership pool power, etc. The situations where there aren't better options to using Black Hole are few and far in between.
Dark Servant is great, he debuffs, he controls, he heals. But, like controller pets, he can't be ordered around like a mastermind's minion, so yeah he's going to engage anything he gets within range of.
Dark Pit is an AoE minion stun, so use it for that. I believe it's accuracy is a little lower than other powers, but it's a solid power if you have room for it. In the past I would generally skip it, but that was before inherent fitness , etc.