I believe that if someone sends you a private message, they cannot block you from returning a message - at least on that exchange.
As long as you "reply" to his message it should go through.
If you are sending him a new message then it could be blocked. At least that is the way it used to work.
Barney
2004 Sunnybrook Titan 30FKS TT
Hensley "Arrow" 1400# hitch (Sold)
Not towing now.
Former tow vehicles were 2016 Ram 2500 CTD, 2002 Ford F250, 7.3 PSD, 1997 Ram 2500 5.9 gas engine