Due to the ArP nerf that they had in 3.2.2, ArP is still an amazing stat to deal with as Blood however it's no longer best. This is due to the fact that yes, Physically damage wise, ArP will still beat Strength for Blood, but you still have some of your attacks that require you to use AP numbers, and ArP has nothing to do with it.
As such, unlike Combat Rogues, Fury Warriors and Kitty Druids, ArP loses it's power due to the fact it isnt' solely based on physical damage anymore. Also the nerf to the co-efficient is pretty low.
However Ensidia as always doesn't explain eff all about why things work the way they do and that's why I really don't like their guides.
ArP Is by far the better stat for Blood however the sheer fact remains that a Blood DK relies on magic attacks as well as physical damage which then decreases the utility of ArP. As such, it is better to go with STrength due to the fact it will increase all damage and not just around 75% of your damage.
Leave it to Ensidia to not explain stuff.