Here is the full list of all 12 Blood Knights achievements.
Revive your co-op partner 10 times
Play through any act with a co-op partner
Kill your co-op partner with an explosive barrel twice
Roya stayed unharmed collecting the vampire blood
All Blood Coins collected
3 vampiric decisions made
Blood Seal returned
3 human decisions made
Kill 10 enemies by pushing them into a pit
Suck 1000 litres of blood
Kill one of your former friends
Kill 10 enemies with an explosive barrel