Always worth keeping up. More dps is always a good thing and blood price should bring in plenty of mana to never be mana starved.

Most of your mana goes towards Dark Arts anyways, aside from Unleash and Unmend. Dark Passenger is on a cooldown and Abyssal Drain is only useful on 4+ enemies (neither of those are worth it atm). So what exactly is there to use your mana on?