So, from what I can see from a cursory glance, blood knights are very elite and highly concentrated force. If you get the charge in they do great, but can get bogged down quite easily.
Compared to other legions/builds, you'd lack healing, respawn, and board coverage/control.
I therefore find myself thinking that point for point, you're better off with 10 black knights (maybe even in 2 units) than 5 blood knights.
Thoughts? Counterpoint? ?