My understanding of the BLM's role in PT is nuke and help the main heal. I've always stated that I won't cure unless it's a nasty AoE and the WHM or RDM cures the tank while I heal everyone else. They are solely responsible for the tank unless they have a bit of time to help with the rest. If I had to help outside of curing, it'll be negating the status effects. If not fighting the mob and we're looking pretty hurt then it's my responsiblity to DS + curaga II while the main healer is resting.
Just like most of you that are pro-/whm, I gotta have that as a subjob for the reason for previous party failures I learned from in the past.