If you persist with Darkness build, next 10 points are going into madness to go up to chain shock.
I actually found while levelling that going Dark Embrace at 41 and 42 was much better for clearing trash rapidly. You can then start in stealth and go Shock -> Wither -> Discharge -> Thrash -> Shock -> Force Lightning without having to pause at all. Soon as you finish channeling you have enough to Wither then Shock again before needing to Saber Strike a bit (if anything is even alive). After that you settle down into the standard rotation - I actually still use this for doing BH\Section X dailies (did not take Torment) as it allows for more rapid burst rather than sustained cost reduction which you would require in boss fights.