DRK needs re-tuning/buffing.

[RaidPanties]

As someone who parses pretty high in susano/lak... this ability feels like garbage to use. For the most part, to me it's a "well I don't have a defensive and I got a lil mana to spare" button. I rarely use Bloodspiller without DA unless I'm about to cap on blood during blood weapon.
The 400 potency is just a lil better than DA syphon strike and worse than a DA Souleater for the same mana cost essentially. Not only this, it further delays my mana regen from Syphon strike which makes it feel like even more of an ass ability to press. Maybe giving it another effect or more damage and I'd feel like it's worth losing the 140 potency potency of Syphon strike?

Other than that TBN is fine, it's single target and aoe blood usages are lackluster and boring to press.

You're severely undervaluing TBN, and your mana management may need a little work, based on limited context clues. You should never have "a little mana to spare sometimes", rather, you should always have the mana to spare a DA or TBN, considering 4800 is the sweet spot for DRKs where you're still doing overall max DPS and are able to respond to C&S coming off CD or baiting a TBN, and not overflow when Delirium comes up for BW.

That said, TBN is a flat DPS positive since Bloodspiller's potency was increased, especially since you can hold the blood you've bought until you can comfortably churn out a DA for it so long as you properly plan ahead to resource dump before TBN'ing, say, Susano's Stormsplitter. If you're using Bloodspiller raw, then sure, it's a loss - but you shouldn't be, unless you severely messed up your gameplay and need to dump blood fast, as wasting blood is even worse.

tl;dr TBN is a great defensive and offensive boon to overall DRK DPS when used and planned around correctly.