Keeping Blood Runes for using Blood Boil single target is always a DPS lost, doesn't matter if you play BoS or Defile. If you are not a retarded Blood DK (most likely 99% of all Blood DKs are) you can manage your Runes with Blood Tap so that you can always use your Skills like Rune Tap, Defile and Soul Reaper with FrostDeath-Runes and UnholyDeath-Runes instead of BloodDeath-Runes. This way you can keep both Blood Runes as Death Runes and maintain higher DPS and better Survivability.
Nevertheless it's much easier to spend for example BloodDeath Runes for Soul Reaper and exactly after use Blood Tap to make them Deaths again, but of cause you need to know how and not fuck it up.
All in all, this Playstyle is much more rewarding than you can imagine but it needs some practice and a really good rune overview as well as PL, BT and ERW Management.