Correct thread sealant?
I asked this same question recently while picking up some supplies at the local plumbing shop. I do general maintenance work, some plumbing, some electrical, carpentry, etc. for a local parks department.
When I asked the question about the proper sealant for different materials, i.e. PVC coupled with iron pipe, five different contractors, in the same breath, said "Blue Blocker." I think it is a dollar or two more but generally the same price as the yellow.
I figured if all of them gave the same answer at the same time, and they do it every day, I went with their advice.
If it's worth doing, it's worth doing right. If you wouldn't put your name on it, it ain't done right!