Things I have learned while dealing with the black tank over many years:
#1. Always treat the cap like a loaded gun. Never stand or put any part of your body in front of it while taking the cap off. EVER!
#2. Always act like there is raw sewage between the slide valve and the cap.
#3. Put the open end of the hose under the cap outlet so even if it has a few drops of goodness it falls in the hose and not on the ground.
#4. Always have a gallon of bleach around in case something happens.
That's my advice and I'm sticking to it!