I can't believe that people regularly switch Grit on and off in line with the cooldowns of Blood Weapon and Blood Price, given that Grit costs MP and a GCD.
Giving these a shared cooldown wouldn't negatively impact the job much at all besides when you transition right as the effect of one of these finishes. You might try to time it like that to take advantage of the effect, but it'd be incredibly minor.
That or they should do away with these skills entirely and have them integral effects of Darkside/Grit that are constant but only 1/3 as effective.