Reddit's D1zz1 has confirmed that you cannot achieve a negative rank in Brothers of Blood in Dark Souls 2.
This is the process he used to conclude that this is indeed the case:
- I lost 10 duels with my normal build, guaranteeing my rank would either be negative or 0.
- Then I soul vesseled and upgraded my character to a fotm cheese build.
- I spent several hours in the arena with my super easy build and earned a net 33 wins.
- I attached a memory scanner/editor to the Darl Souls 2 process and found the address that stores the value for the blood rank.
- I confirmed it by also testing changes in the values for the other covenants.
- At this point that value should either have been 33 or negative by a wide margin. It was 33. Meaning rank cannot go below 0.
He says he was frustrated by vague claims that the score could drop below zero and so decided to investigate, you can check out his full remarks here.
Earlier this week, a three part DLC titled The Lost Crowns was announced for Dark Souls 2 and the episodes will be released between July and September.