I've applied Midnight Sun over BFWD after just about an hour and also after 12 hours, no difference as far as I could tell in feel or looks. Here is a quote (copy and pasted) from Todd "th0001", the moderator of TID about Blackfire Wet Diamond:
"To be honest, you could probably apply another product over BFWD an hour or two after application. Since we injected BFWD with Polycharger (a catalyst that dramatically decreases the curing time) it is ready to go almost instantly. The problem is that the directions/instructions have to be written so that everybody gets consistent results.
Applying BFWD in extreme humidity, extreme cool tempatures, or massive overapplication (putting it on super thick) will all contribute to longer than normal cure times. Doing all three together (globbing it on a cold, humid day) could require 8-12 hours before it is fully cured."
BTW, the Blackfire shampoo is my favorite car wash out of the dozen+ I've tried yet. After trying a 16oz bottle I bought a gallon.