Stealth is important for Duskblade to work
Had 2 Riven in different games try a lethality build, they won lane really well but as soon as they grouped up they were useless.
Veigar was intentionally feeding so we lost but Riven was insanely ahead: https://image.prntscr.com/image/BP8LMuHHSeyil63vgyjCPw.png
Once again a super ahead Riven, she was 6/1 by 13 minutes but still, lost the team fights: https://image.prntscr.com/image/7uJ_0A7VRwOGVVUda700Bw.png https://image.prntscr.com/image/4jKhTy90Sm_RDzt2b4GosA.png
So yeah. Riven never used the actual duskblade proc in teamfights because we would have vision of her. Duskblade is strong and all but you gotta make sure you get that stealth otherwise it won't be good. Also gotta ask yourself if you have enough mobility to get to the enemy and proc it within 1s because that's how long it will last if you are seen.
In that second game Riven also got an insane 4 man knock up during that fight, her team was right behind her as well. We still won though because she just didn't have enough with lethality alone to actually one shot. I also did a clutch exhaust :s but still.
Before you go on and buy duskblade on everyone make sure you can actually use it effectively or that the lethality will be enough for you.