Hi, I am not sure what your getting at but, tongs are not people. They know tongs as tongs, in fact, most monitors react very strangely to tongs. Specially the metal ones. They act like it shocks them.
They know you are at the other end of the tongs. So them eating off tongs only means they are only allowing you to a certain distance. I have no doubt if you held food in your hands they will quickly learn to eat out of your hands.
About how fast is not a good question. I have seen wildcaught backtrees, eat out of my hands right out of the shipping box(funny story)
In my experience blacktrees are very normal monitors. They adjust well and are very curious. Mine would see me coming and look at my hands, If I had food, they would climb out to the edge of the limbs and beg for food. If I did not have food, they would look discouraged and go back to what they were doing.
Anyway, its about your conditions and their condition. Their conditions before you got them, etc. I would think your doing well. I would think your female had problems before you got here. As I saw no differences with males and females as far as eating off tongs.
Also, its about individuals, some are just nuts, both ways. Some do not like people. Again, something may have influenced that. Remember, someone collected them. The folks that collect them, do not treat them how you would treat them. In fact, they treat them as nasty things. Normally tying them up. Normally by the hips and then tying them to a post or tree until they are picked up. Anyway, it can be very traumatic.
So do not expect them to be normal or consistant.
Oh wait the other way, I have an ackie that leaps up in the air every single time he sees me. Every once in a while I hold out a mouse, yes a mouse, and he leaps up and gets it. So he thinks its great to leap. But if I go and grab him or try to pick him up in anyway. He acts like that is not part of this game. hahahahahahahaha. It bugs him, but not much more.
Anyway, to judge how your monitors are doing by how they eat off tongs is a little, how can I be nice, naive. Eating off tongs is a good sign but not all that meaningful. Gaining weight, growing, cycling, etc, are real tangible signs. All of those need support to occur. Even if they do not eat in front of you or do not feed off tongs. Feeding off tongs depends on to many things. Cheers
I mentioned conditions being important. If your conditions are good, its reflected in a feeding responce. If they have a superior feeding responce, they will eat off tongs, eat the tongs, eat the hands connected to the tongs, etc.
A year or two ago, I posted a pic of a whitethroat eating a coke can. I was drinking that dang coke and accidently dropped it into the cage. The monitor bolted out from under something and grabbed the can and ran off. It then learned it was not edible. I wanted a pic so I went to get the camera. Of course not he would not do it again. So I rubbed a mouse on the can and he grabbed it and I got my pic. hahahahahahahahahaha
Cheers