Well not sure how the digbit admin correlates chrome with attackers but here is hes explaination for chrome user-agent not working:
Yes, it only limit chrome Chrome/13.0.782.109, because attack are using this.
LOL , take your pick guys
http://www.user-agents.org/Sorry, I know I'm a few pages behind and this may have already been suggested:
Couldn't we have a list of UAs that are randomly (or round robin-ly) rotated through for each lookup request so that there isn't a single detectable signal from BH? If we have all the main browsers (IE/Safari/Firefox/Chrome/Opera) on several platforms (Windows 7/XP/Mac OS/iPhone/Ubuntu/Android) it would make it impossible for a pool operator to identify BH traffic from any other traffic.