Dash Hope presents other players with the choice of losing life or letting their spell get countered, at the same cost as [[Counterspell]]. What's the drawback?

Temporal Extortion presents other players with the choice of losing half their life or letting the caster take an extra turn, at a similar cost to other extra turn spells (commonly 3UU vs BBBB). What's the drawback?

Divination and Painful Lesson cost the same, Omnipotence and Omniscience do not, you even discounted Omnipotence.

I will agree that black is capable of breaking the color pie however, in each of the instances you mention, the black card does the same effect for the same or similar cost as it's counterpart.