I'm not sure how mechanically it works, but I would assume it goes like this...
step 1 - Self-buff, make awesome - outbreak on boss (snapshot 1 @ awesome)
step 2 - Pestilence off boss to adds (snapshot 2 @ not as awesome)
step 3 - Mess with adds, pestilence off adds that hits boss (snapshot 3 @ probably meh)
result - Boss now has snapshot 3 on him, thus screwing up festerblight
Whenever diseases are placed on a target, it creates a new snapshot of your current str/crit/haste/etc. Rolling Blood would cause steps 2 and 3 to happen almost simultaneously if you're not careful.
Basically, as long as the primary target is never over-written, it wouldn't be an issue. Rolling blood would cause everyone to spreak the diseases, thus overwriting the primary target. The note is to be careful that you Don't over-write your festerblight target's diseases, and only use pestilence off him.