I dont use the dart technique for jumping. I get down on the shot with the jump cue similar to a normal shot and site the shot through the blocking ball. I then just raise up in the position I am in for a jump shot and aim for the spot on the blocking ball that the line went though. The blocking ball ends up being much easier to see then the object ball when jacked up.
Works well for me.
This is also the way I aim for jumps. It's surprisingly accurate