I have been tieing loops for years and never had one slip on me.
Ensure the knots are opposing each other and use alot of tension (I use Viper Pliers) to tighten them.
I do not use black rope either, it stretches more than any other color out there for some reason. Some of the best I have seen is the Paradox Cool Spring release rope Cabelas sells, stuff is super stiff and GREAT quality; seems like it is built to last.




Here is more aggressive way to tie a triple loop if needed, I have never domne this but, I am sure it will work.