They might do well in a 2k game. Points will get a little tight for others but it seems Frenzy works on mounts too. On the charge a single Blood Knight has A/3 at S/7 with a mount who tosses in A/2 at S/4.
Still believe the unit is a bit risky with the one bad war machine shot or spell breaking this army backbone unit.
Also if you do use them bring some Direwolves to help control their uncontrolled charges. Two units of Wolfies block the vampires from charging until all three are ready to charge the same target. Split frontage will bring the Blood Knights into the main part of the unit with the Dirwolves on each side.
A five model unit charging most enemies will hit on 3+, A/15 (or A/16 with champion), at S/7 (-4 to Armor Saves) plus the horses 4+, A/10, at S/4. Cruel against an infantry unit, painful for an armored unit.