I have tested on the test server and I am telling you that the only thing a bk can beat is a elf. Esm kills anything, RF has the potential to kill everything but lacks range, SMG is balanced pretty well between damage taken and given, DL does pretty good, Summoners do good against anything, and the only thing elf has is ice arrow.
I tried ebk, abk, sbk, hybrid str ene, and hybrid str agi. All builds with 10k to 13k vit. Best so far was agi bk which could survive but couldnt do enough damage to win. I did the tests 2 ways : all out combo, and spamming twisting for stun then combo.
Because bk lacks any ranged attack the damage should be better. I landed 7 consecutive combos on an sm and was not able to kill.