If the target of your regen spell has boil blood on them you can not refresh the regen or apply it to them.
Lava Duct Crawlers (LDC) apply boil blood and this is how I learned about this.
This applies to regeneration, chroloplast and regrowth or any of the group based ones.
Edit: going to place this link here. as Boil Blood is not officially confirmed to be accurate on p99 in terms of Resistance Check
http://www.project1999.com/forums/sh...d.php?t=228438