This is the knot that I use for joining main line to shockleader. It is very strong and I can honestly say I have never had any problems with it coming apart.
The big advantage of the blob knot is that you can use large diameter shockleader line as you do not have to pull a knot tight in it. Simply touch the end of the shockleader into the flame of a lighter until it melts into a small blob. (Not too big).
Then simply tie main line as shown. Plenty of saliva and pull tight. Make sure you pull main line tight up against the blob of the shockleader before cuting off tag.
Design by Jaybee